MNG
1.0
MNG (Multiple-image Network Graphics) Format Version 1.0
For list of authors, see
Credits (Chapter 19)
Status of this Memo
This document is a specification by the PNG development group.
It has been approved by a vote of the group. Future technical
changes will require formal approval by a vote of the group. It is the
intent of the group to maintain backward compatibility if possible.
Comments on this document can be sent to the MNG specification
maintainers at one of the following addresses:
mng-list@ccrc.wustl.edu
png-group@w3.org
png-info@uunet.uu.net
Distribution of this memo is unlimited.
At present, the latest version of this document is available on the
World Wide Web from
ftp://swrinde.nde.swri.edu/pub/mng/documents/
Abstract
This document defines the MNG
(Multiple-image Network Graphics) format.
It also defines the MNG-LC (Low Complexity), MNG-VLC (Very Low Complexity),
and JNG (JPEG Network Graphics) formats.
These are proper subsets of MNG.
MNG is a multiple-image member of the PNG
(Portable Network Graphics) format family. It can contain animations,
slide shows, or complex still frames, comprised of
multiple PNG
or JNG
single-image datastreams.
The MNG and JNG formats use the same chunk structure that is defined
in the PNG specification, and they share other features of the PNG
format. Any MNG decoder must be able to decode PNG and JNG datastreams.
The MNG format (but not MNG-LC or MNG-VLC)
provides a mechanism for reusing image data without
having to retransmit it. Multiple images can be composed into
a "frame"
and a group of images can be used as an animated "sprite" that
moves from one location to another in subsequent frames. "Palette
animations" are also possible. MNG can also
store images in a highly compressible "Delta-PNG" format,
defined herein.
A MNG
frame normally contains a two-dimensional image or a
two-dimensional layout of smaller images. It could also contain
three-dimensional "voxel" data arranged as a series of
two-dimensional planes (or tomographic slices), each plane being
represented by a PNG
or
Delta-PNG datastream.
A Delta-PNG datastream defines an image in terms of a parent PNG
or Delta-PNG image and the differences from that image. This
provides a much more compact way of representing
subsequent images than using a complete PNG datastream for each.
This document includes examples that demonstrate various capabilities
of MNG. These include simple movies, composite frames, loops, fades, tiling,
scrolling, storage of voxel data, and converting GIF animations to MNG
format.
Reading this document
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Table of Contents
1. Introduction
2. Terminology
3. Objects
3.1. Embedded objects
3.2. Object attributes
3.3. Object buffers
3.4. Object 0
4. MNG Chunks
4.1. Critical MNG control chunks
4.1.1.
MHDR
MNG datastream header
4.1.2.
MEND
End of MNG datastream
4.1.3.
LOOP, ENDL
Define a loop
4.2. Critical MNG image defining chunks
4.2.1.
DEFI
Define an object
4.2.2.
PLTE and tRNS
Global palette
4.2.3.
IHDR
, PNG chunks,
IEND
4.2.4.
JHDR
, JNG chunks,
IEND
4.2.5.
BASI
, PNG chunks,
IEND
4.2.6.
CLON
Clone an object
4.2.7.
DHDR
, Delta-PNG chunks,
IEND
4.2.8.
PAST
Paste an image into another
4.2.9.
MAGN
Magnify objects
4.2.10.
DISC
Discard objects
4.2.11.
TERM
Termination action
4.3. Critical MNG image displaying chunks
4.3.1.
BACK
Background
4.3.2.
FRAM
Frame definitions
4.3.3.
MOVE
New image location
4.3.4.
CLIP
Object clipping boundaries
4.3.5.
SHOW
Show images
4.4.
and
SEEK
chunks
4.4.1.
Save information
4.4.2.
SEEK
Seek point
4.5. Ancillary MNG chunks
4.5.1.
eXPI
Export image
4.5.2.
fPRI
Frame priority
4.5.3.
nEED
Resources needed
4.5.4.
pHYg
Physical pixel size (global)
4.6. Ancillary PNG chunks
5. The JPEG Network Graphics (JNG) Format
5.1. Critical JNG chunks
5.1.1.
JHDR
JNG header
5.1.2.
JDAT
JNG image data
5.1.3.
IDAT
JNG PNG-encoded alpha data
5.1.4.
JDAA
JNG JPEG-encoded alpha data
5.1.5.
JSEP
8-bit/12-bit image separator
5.1.6.
IEND
End of JNG datastream
5.2. Ancillary JNG chunks
6. The Delta-PNG Format
6.1. Delta-PNG critical chunks
6.1.1.
DHDR
Delta-PNG datastream header
6.1.2.
IDAT, JDAT
, and
JDAA
New pixel data
6.1.3.
PROM
Promotion of parent object
6.1.4.
IHDR
PNG image header
6.1.5.
IPNG
Incomplete PNG
6.1.6.
PLTE
and
tRNS
6.1.7.
PPLT
Partial palette
6.1.8.
JHDR
JNG image header
6.1.9.
IJNG
Incomplete JNG
6.1.10.
DROP
Drop chunks
6.1.11.
DBYK
Drop chunks by keyword
6.1.12.
ORDR
Ordering restrictions
6.2. Ancillary Delta-PNG chunks
6.2.1.
gAMA, cHRM, iCCP, sRGB
Color space chunks
6.2.2.
oFFs
and
pHYs
6.2.3. Other ancillary PNG chunks
6.2.4.
IEND
End of Delta-PNG datastream
6.3. Chunk ordering requirements
7. Extension and Registration
8. Chunk Copying Rules
9. Minimum Requirements for MNG-Compliant Viewers
9.1. Required MNG chunk support
9.2. Required PNG chunk support
9.3. Required JNG chunk support
9.4. Required Delta-PNG chunk support
10. Recommendations for Encoders
10.1. Use a common color space
10.2. Use the right framing mode
10.3. Immediate frame sync point
10.4. Embedded images in LOOPs
10.5. Including optional index in SAVE chunk
10.6. Interleaving JDAT, JDAA, and IDAT chunks
10.7. Use of the JDAA chunk
11. Recommendations for Decoders
11.1. Using the simplicity profile
11.2. ENDL without matching LOOP
11.3. Note on compositing
11.4. Retaining object data
11.5. Decoder handling of fatal errors
11.6. Decoder handling of interlaced images
11.7. Decoder handling of palettes
11.8. Behavior of single-frame viewers
11.9. Clipping
12. Recommendations for Editors
12.1. Editing datastreams with optional index
12.2. Handling LOOP and TERM chunks
13. Miscellaneous Topics
13.1. File name extension
13.2. Internet media type
13.3. EBNF Grammar for MNG, PNG, and JNG
13.4. Uniform Resource Identifier (URI)
14. Rationale
15. Revision History
15.1. Version 1.0
15.2. Version 0.99
15.3. Version 0.98
15.4. Version 0.97
15.5. Version 0.96
15.6. Version 0.95
16. References
17. Security Considerations
18. Appendix: Examples
18.1. Example 1: A single image
18.2. Example 2: A very simple movie
18.3. Example 3: A simple slideshow
18.4. Example 4: A more storage-efficient slideshow
18.5. Example 5: A simple movie
18.6. Example 6: A single composite frame
18.7. Example 7: A movie with sprites
18.8. Example 8: A movie with an animated sprite
18.9. Example 9: "Fading in" a transparent image
18.10. Example 10: Storing three-dimensional images
18.11. Example 11: Tiling
18.12. Example 12: Scrolling
18.13. Example 13: Cycling animations
18.14. Example 14: Converting a GIF animation
18.15. Example 15: Converting a simple GIF animation
18.16. Example 16: Counting layers and frames
18.17. Example 17: Storing an icon library
18.18. Example 18: MAGN methods
18.19. Example 19: MAGN chunks and ROI
19. Credits
1. Introduction
This specification defines the format of a MNG (Multiple-image Network
Graphics) format.
It also defines low-complexity and very-low-complexity versions (MNG-LC and
MNG-VLC), and the JNG (JPEG Network Graphics) format, which are proper subsets
of MNG.
Note: This specification depends on the PNG (Portable Network
Graphics)
[PNG]
and the JPEG (Joint Photographic Experts Group)
specifications.
The PNG specification is available at the PNG web site,
MNG is a multiple-image member of the PNG format family that can contain
animations,
slide shows, or
complex still frames,
comprised of multiple PNG
or JNG
single-image datastreams.
Like PNG, a MNG datastream consists of an 8-byte signature, followed
by a series of chunks. It begins with the
MHDR
chunk and ends
with the
MEND
chunk. Each chunk consists of a 4-byte data length
field, a 4-byte chunk type code (e.g., "MHDR"), data (unless
the length is zero), and a CRC (cyclical redundancy check value).
A MNG datastream describes a sequence of zero or more single frames,
each of which can be composed of zero or more embedded images or directives
to show previously defined images.
The embedded images can be PNG, JNG, or Delta-PNG datastreams.
MNG-LC and MNG-VLC datastreams do not contain JNG
datastreams, but MNG-LC and MNG-VLC
applications can be enhanced to recognize and process JNG datastreams as well.
A typical
MNG
datastream consists of:
The 8-byte MNG signature.
The
MHDR
chunk.
Frame definitions. A frame is one or more layers,
the last of which has a nonzero interframe delay,
composited against whatever was already on the display.
Layer definitions.
An embedded potentially visible image, described by PNG or JNG
datastreams
or the MNG
BASI
chunk
(a foreground layer).
An image that is generated from a stored object as directed by certain
MNG chunks (a foreground layer).
The background (a background layer).
LOOP-ENDL
chunks.
SEEK
chunks that mark points in the datastream where
processing can be restarted.
Various chunks for creating and manipulating images and other
objects.
The
MEND
chunk.
MNG is fundamentally declarative; it
describes the elements that go into an individual frame. It is up
to the decoder to work out an efficient way of making the screen match
the desired composition whenever a nonzero interframe delay occurs. Simple
decoders can handle it as if it were
procedural, compositing the images into the frame buffer in the order
that they appear, but efficient decoders might do something different,
as long as the final appearance of the frame is the same.
Images can be "concrete" or "abstract". The
distinction allows
decoders to use more efficient ways of manipulating images when it
is not necessary to retain the image data in its original form or
equivalent in order to show it properly on the target display system.
MNG is pronounced "Ming."
When a MNG datastream is stored in a file, it is recommended that
".mng" be used as the file suffix. In network applications, the
Media Type "video/x-mng" can be used. Registration of the media
type "video/mng" might be pursued at some future date.
The MNG datastream begins with an 8-byte signature containing
138 77 78 71 13 10 26 10 (decimal)
8a 4d 4e 47 0d 0a 1a 0a (hexadecimal)
\212 M N G \r \n \032 \n (ASCII C notation)
which is similar to the PNG signature
with "\212 M N G"
instead of "\211 P N G" in bytes 0-3.
MNG does not yet accommodate sound or complex sequencing
information, but these capabilities might be added at a later date, in
a backward-compatible manner. These issues are being discussed in the
mng-list@ccrc.wustl.edu
mailing list.
Chunk structure (length, name, data, CRC) and the chunk-naming system
are identical to those defined in the PNG specification. As in PNG, all
integers that require more than one byte must be in network byte order.
The chunk copying rules for MNG employ the same mechanism as PNG,
but with rules that are explained more fully
(see
below, Chapter 8
).
A MNG editor is not permitted to move unknown chunks across
the
and
SEEK
chunks, across any chunks that can
cause images to be created or displayed, or into or out of a
IHDR-IEND
or similar sequence.
Note that decoders are not required to follow any decoding models
described in this specification nor to follow the instructions in this
specification, as long as they produce results identical to those that
could be produced by a decoder that did use this model and did follow
the instructions.
Each chunk of the MNG datastream or of any embedded object is an
independent entity, i.e., no chunk is ever enclosed in the data segment
of another chunk.
MNG-compliant decoders are required to recognize and decode independent
PNG or JNG datastreams.
Because the embedded objects making up a MNG are normally in PNG
format, MNG shares the good features of PNG:
It is unencumbered by patents.
It is streamable.
It has excellent, lossless compression.
It stores up to four channels (red, green, blue, alpha), with up to
16 bits per channel.
It provides both binary and alpha-channel transparency.
It provides platform-independent rendition of colors by inclusion of
gamma and chromaticity information.
It provides early detection of common file transmission errors and
robust detection of file corruption.
Single-image GIF files can be losslessly converted to PNG.
It is complementary to JPEG and does not attempt to replace JPEG
for lossy storage of images (however,
MNG can accommodate JPEG-encoded
images that are encoded in the PNG-like JNG
format that is defined herein).
In addition:
It provides animation with
variable interframe delays.
It allows composition of frames containing multiple images.
Using JPEG compression together with a magnification factor, it can
achieve 1000:1 and higher lossy compression of Megapixel
truecolor images. While some detail is lost, such highly-compressed
images are useful as full-scale previews and for layout work.
It facilitates the use of images as "sprites" or groups of
images as "animated sprites" that can be reused in subsequent
frames.
It capitalizes on frame-to-frame similarities to reduce the amount
of data that must be included in a datastream.
It provides "restart" points at which processing can be
safely resumed in case of data loss or corruption, or to which applications
can jump if they have random access to the file.
A "frame priority" chunk allows authors to indicate which frame
should be displayed by single-image viewers, and a subset of the frames
that should be displayed by slow viewers.
Images and frames can be given names, allowing authors to mark them
for export outside the scope of MNG, where they can be used for icons or
similar purposes.
A series of PNG and JNG images can be losslessly converted to MNG
and back to a series of equivalent PNG or JNG images, even when the
delta format is used to store them in the MNG.
JNG provides JPEG with alpha-channel transparency and color space
information.
Multiple-image GIF files
can be losslessly converted to MNG, and,
(except for those using
the "restore-to-previous" disposal method)
can be losslessly converted to
MNG-LC and (except for those with a variable framing
rate, and less efficiently, except also for those using the
"restore-to-background" disposal method) to MNG-VLC.
Most JPEG files can be losslessly converted to JNG or MNG, and all
JNG datastreams can be losslessly converted to JPEG files.
It is complementary to MPEG and does not attempt to replace MPEG for
lossy storage of video. MNG does, however, provide the capability of
storing animations consisting of JPEG-encoded images that have been
wrapped in the JNG format.
2. Terminology
See also the glossary in the PNG
specification.
requirement levels
The words "MUST", "MUST NOT",
"REQUIRED",
"SHOULD", "SHOULD NOT",
"RECOMMENDED", and "OPTIONAL" in this document,
which are to be interpreted as
described in
RFC-2119
The word "CAN" is
equivalent to the word "MAY" as described
therein. "NOT ALLOWED" and
"NOT PERMITTED" describe conditions
that "MUST NOT" occur. "ALLOWED"
and "PERMITTED" describe conditions that "CAN" occur.
abstract image or object
An image whose pixels have a private representation, and which does
not necessarily carry PNG or JNG chunk data. An image delta cannot be
applied to an abstract image. All abstract objects are viewable.
Object 0 is always abstract, since it is never stored.
animation
A sequence of images meant to be played at a framing rate that will
give the impression of motion. We use the more generic
term "sequence"
to include any group of images meant to be played at some specified
framing rate or under user control, not necessarily an animation, such
as a slide show, as well as animations.
cheap transparency
Image transparency data conveyed via the PNG
tRNS
chunk rather
than via a full alpha channel.
child, or child image
An image produced by applying an image delta to a parent object.
clipping boundaries
Limits within which a pixel must fall to be displayed.
The left and top boundaries are inclusive, while the right and bottom
boundaries are exclusive.
color encoding
File gamma and chromaticity values, an sRGB rendering intent, an
iCCP profile, or whatever is involved in mapping between RGB values and
colors.
concrete image or object
An image or object whose pixels have a publicly known representation,
and which uses a publicly known color encoding. A concrete PNG or JNG
image also carries data from other known PNG or JNG chunks that are
present.
embedded object or image
A concrete object or image that appears in-line in a MNG datastream.
frame
A composition of zero or more layers that have zero interframe delay time
followed by a layer with a specified nonzero delay time or by the
MEND
chunk. A frame is to be displayed as a still picture or as part of a sequence
of still images or an animation.
An animation would ideally appear to a perfect observer (with an inhumanly fast
visual system) as a sequence of still pictures.
In MNG-VLC datastreams, each frame (except for the first, which also
includes the background layer) contains a single layer,
unless the framing rate (from the
MHDR
ticks_per_second
field) is zero. When the framing rate is zero,
the entire datastream describes a single frame.
When the
layers of a frame do not cover the entire area defined by the width and
height fields from the
MHDR
chunk, the layers
are composited over the previous frame to obtain the new frame.
When the frame includes the background layer, and the
background layer is transparent, the transparent background is composited
against the outside world and the subsequent layers are composited against
the result to obtain the new frame.
frame origin
The upper left corner of the output device (frame buffer, screen,
window, page, etc.) where the pixels are to be displayed.
This is the
{0,0} position for the purpose of defining frame clipping boundaries,
image locations, and image clipping boundaries.
Note that in a
windowing system, the frame origin might be moved offscreen, but
the locations in
DEFI
MOVE
, and
CLIP
chunks
would still be measured from this offscreen origin.
In MNG-VLC, all images must be placed with the image's upper left
corner at the frame origin.
framing rate
The rate, measured in frames per second, at which frames are displayed
on the output device. In a MNG datastream, the framing rate is
the interframe delay, in ticks, divided by the number of ticks per second,
from the
MHDR
chunk.
The
FRAM
chunk can be used to change the framing rate for a portion
of the datastream.
frozen object
An object whose set of object attributes and whose object buffer are not
allowed to be discarded, replaced, or modified.
image delta
An object that can be applied to a concrete image or object to
produce another concrete image. For any two concrete images, there
exists an image delta that will produce one from the other.
image N or object N
Shorthand for "the object with the set of object attributes pointed
to by
`object_id=N'
".
In MNG-LC and MNG-VLC, only image 0 is permitted.
interframe delay
The amount of time a layer should be visible when a sequence of frames
or an animation is played. A layer with a zero interframe delay is
combined with the subsequent layer or layers to form a frame; the frame
is completed by a layer with a nonzero interframe delay or by the
MEND
chunk. In reality, it takes a nonzero amount of time to display a
frame. No matter which moment is picked as the "start" of
the frame, the interframe delay measures the time to the "start"
of the next frame. There is no interframe delay prior to the implicit
background layer at the beginning of the sequence nor after the final
frame.
interpolate
To determine the color or alpha values for new pixels
that have been created in the interval between two pixels with known values.
In this document, interpolation always means linear interpolation (the new
values are evenly spaced between the two known values).
iteration
One cycle of a loop. In this document, as is customary among
computer programmers, the number of iterations of
a loop includes the first cycle. A loop can have zero iterations,
which means it is not executed at all.
layer
One of
A visible embedded
image,
located with respect to
the frame boundaries and clipped with respect to the layer clipping
boundaries and the image's own clipping boundaries.
A stored image that is displayed in
response to a
SHOW
CLON
, or
MAGN
chunk directive,
located and clipped.
The background that is displayed before the
first image in the entire datastream is displayed. Its contents can be defined
by the application or by the
BACK
chunk.
The background image, clipped, located, and displayed against
a solid rectangle filled with the background color and clipped to the
subframe boundaries, that is used as a background
when the framing mode is 3 or 4.
Note that a layer can be completely empty
if the image is entirely outside the clipping boundaries.
A layer can
be thought of as a transparent rectangle with the same dimensions as
the frame, with an image composited into it, or it can be thought of as
a rectangle having the same dimensions (possibly zero) and location as
those of the object after it has been located and clipped.
The layers in a MNG datastream are gathered
into one or more subframes for convenience in
applying frame parameters to a subset of the layers
(see the definition of "subframe"
below
).
An embedded visible PNG or JNG datastream generates a
single layer,
even though it might be interlaced or progressive.
If the background
consists of both a background color and a background image, these
are combined into a single layer.
MNG-LC
A low-complexity subset of MNG that does not use stored object
buffers or certain other complex features. The "simplicity profile"
in the
MHDR
chunk must meet certain requirements
(see the
MHDR
chunk specification
below, Paragraph 4.1.1
).
MNG-VLC
A very-low-complexity subset of MNG that does not use stored objects,
variable framing rates, location of images at positions other than (0,0),
or other complex features. The "simplicity profile"
in the
MHDR
chunk must meet certain requirements
(see the
MHDR
chunk specification
below, Paragraph 4.1.1
).
nullify
To nullify a chunk is to undo its effect, restoring the datastream to
the condition it would have had if the chunk being nullified had never
appeared.
object, object_id
An image or a nonviewable basis object.
The
object_id
is
an unsigned sixteen-bit number that serves as the identifier of a set of
object attributes. In MNG-LC
and MNG-VLC
only object 0 is permitted.
object attributes
Properties of an object such as its existence, potential visibility,
location,
clipping boundaries, and a pointer to an object buffer.
See
Object attributes
below.
object buffer
A 2D array of pixels or pixel deltas, each of which has color and
transparency information. More than one object can point to a given
object buffer. See
Object buffers
below.
parent, parent object, or parent image
An object to which a delta is applied.
pixel sample depth and alpha sample depth
The sample depth used for decoding
IDAT
data
in Delta-PNG and JNG datastreams and
JDAA
data in JNG datastreams.
They are not necessarily the same as the
sample depth of the object, which is called "sample depth" or
"object sample depth" in this document.
potentially visible image
One of
not-yet-defined object that is "marked",
by
setting its
do_not_show
flag to zero,
for
on-the-fly
display while the embedded image that defines it is being
cloned or
decoded.
an existing object that has been made potentially visible (i.e., it has
been marked for being made visible by subsequent
SHOW
chunks), by
setting its
do_not_show
flag to zero.
prologue segment
The first segment, when there is more than one segment.
regular segment
Any segment other than the first (also the first segment, when there
is only one).
replication
Making an additional copy. If you replicate something
N times, you end up with N+1 of them.
segment
A part of a MNG datastream starting with the
MHDR
chunk
or with a
SEEK
chunk and extending to just before the next
SEEK
chunk (or the
MEND
chunk if there is no next
SEEK
chunk). The
MHDR
MEND
SEEK
, and
TERM
chunks are not considered to be a part
of any segment.
signal
An entity with a number that can arrive asynchronously at the
decoder. More detailed semantics, like whether multiple signals of the
same number (or even different numbers) can be queued, are beyond the
scope of this specification.
subframe
A subset of the layers defined by a MNG datastream, gathered
for convenience in applying frame parameters
(i.e., clipping information, interframe delay,
timeout, termination condition, and a name.
See the definition of "frame"
above
).
The extent of a subframe depends on the framing mode; it can be
a single layer,
the set of layers appearing between
FRAM
chunks,
a background layer and a single foreground layer, or
a background layer plus the set of layers
appearing between
FRAM
chunks.
See the
FRAM
chunk
specification
below (Paragraph 4.3.2)
viewable image
A stored object or embedded object that is capable of being made visible.
An image is viewable, while some objects resulting from decoding a
BASI
datastream are not viewable.
visible image
Actually drawn on a display. If an object is visible, a person
looking at the display can see it.
3. Objects
An "object", which is identified by
an
object_id
, is an
image or it is a nonviewable entity that is created by the
BASI
chunk. The
object_id
is an unsigned sixteen-bit number that
serves as the identifier of a set of object attributes.
An "image" is a viewable object.
Object 0 is a special object whose pixel data is not available for later
use (see
below
).
3.1. Embedded objects
An embedded object is:
A PNG datastream (
IHDR
, PNG chunks,
IEND
).
A JNG datastream (
JHDR
, JNG chunks,
IEND
).
A BASI datastream (
BASI
, PNG chunks,
IEND
).
3.2. Object attributes
Objects have
object attributes
that can be defined
and modified by the contents of various MNG chunks. Decoders are
responsible for keeping track of them. The simplest decoder might
establish a 65,536-element array for each attribute, but real
applications will undoubtedly use a more memory-efficient method.
Object attributes include:
Existence
A nonzero object comes into existence when
DEFI
chunk creates it.
CLON
chunk creates it.
A nonzero object ceases to exist when it does not have
the "frozen" attribute and
it is the subject of a
DISC
chunk.
an empty
DISC
chunk appears.
SEEK
chunk appears.
the
MEND
chunk appears (or the
IEND
chunk appears
in a simple PNG or JNG file).
a new embedded object with the same
object_id
replaces it
without an intervening
DEFI
chunk. In this case, the new object
inherits the set of object attributes from the previous object with the same
object_id
Object 0 always exists.
Pointer to an object buffer
Every object (except for object 0) has an object buffer. Multiple
objects can point to the same object buffer. The representation of a
pointer is decided by the application; pointers never appear explicitly
in a MNG datastream. Decoders can also create an object buffer for
object 0, if that is more convenient, but the information in that buffer
cannot be depended upon to exist after the image has been displayed, nor
can that buffer become "frozen".
Frozen or not frozen
All objects are initially "not frozen". Any objects in
existence (except for object 0) when the
chunk is encountered
become "frozen", along with the object buffers that they point to.
Potential visibility
The "potential visibility" of an object is determined by the
do_not_show
byte of the
DEFI
or
CLON
chunk
that introduced it. The "potential visibility" of viewable objects
can be changed by the
SHOW
chunk. When an embedded object is
"potentially visible," it can be displayed "on-the-fly"
as it is being
decoded. Later, the
SHOW
chunk can direct that a "potentially
visible" viewable object be displayed.
It is permitted to change the potential visibility of "frozen"
objects; if this is done, the potential visibility must be restored to
its "saved" condition by the encoder prior to the end of the
segment.
Viewability.
An object is viewable if it has a viewable object buffer. It is
nonviewable if it has a nonviewable object buffer or if its object buffer
has not yet been created or has been destroyed. Any attempt to display a
nonviewable object must be ignored and not treated as an error.
A nonviewable object becomes viewable immediately when the
decoder receives a viewable object buffer or when an image delta makes it
viewable, and if the object is potentially visible it can be displayed
"on-the-fly" while the object buffer is being decoded or updated.
Note that object 0 is only viewable while its embedded image is being
decoded and displayed on-the-fly, after which it becomes nonviewable
again because no object buffer is ever created for object 0.
Location
The X and Y location of an object is determined by the
DEFI
chunk that introduced it, and can be changed by the
MOVE
chunk.
It is permitted to change the location of "frozen"
objects, provided that the encoder includes a
MOVE
or
DEFI
chunk prior to the end of the segment that restores their locations to their
"saved" positions.
Clipping boundaries
The clipping boundaries of an object are determined by the
DEFI
chunk that introduced it, and can be changed by means of
the
CLIP
chunk.
It is permitted to change the clipping boundaries of "frozen"
objects, provided that the encoder includes a
CLIP
chunk
prior to the end of the segment that restores the boundaries to their
"saved" values.
Additional information
While not required by this specification, applications may wish to
store other information about the object, such as whether it is eligible
to be updated by block-alpha-addition, for error-checking purposes.
3.3. Object buffers
An object buffer is created by the appearance of an embedded object
in the datastream, with a nonzero
object_id
, or by the
appearance of a
CLON
chunk that specifies a "full clone".
The contents of an object buffer can be modified by processing an image
delta or a
PAST
chunk.
Object buffers contain a 2D array of pixel data and can contain
additional information. In addition, decoders are responsible for
keeping track of some properties of the data in the object buffer:
Object 0 conceptually never has an object buffer. Decoding applications
can create one for their own convenience, but such an object buffer must
never be made available to the rest of the MNG datastream or be considered
viewable after it has been processed.
When the "stored object buffers" flag (bit 9 of the simplicity
profile) is 0
and valid (i.e., bit 6 is 1 and bit 9 is 0), an object buffer need not be
created even when an embedded object with a nonzero
object_id
appears, since the flag promises that the object buffer will never be used
again.
There is no requirement
not
to create an object buffer; no
harm will be done except for some unnecessary memory consumption.
Viewability of object buffer
Any object that points to a viewable object buffer can be
displayed, but one that points to a nonviewable one cannot.
Any attempt to do so must be ignored.
A PNG or JNG datastream always has the "viewable" attribute.
The "viewable" attribute of a
BASI
datastream is
defined in the
BASI
chunk. Only
BASI
datastreams that
describe an object equivalent to one described by a legal PNG datastream
can be declared "viewable".
When a Delta-PNG is applied to a parent object, the resulting object
buffer always has the "viewable" attribute.
Format of data in the object buffer
The data format can be:
A concrete PNG or JNG object.
A concrete object must be stored by the decoder in a form that
retains the complete object description, sufficient to regenerate the
original object description or its equivalent without loss. Its pixels
have a publicly known representation and it uses a publicly known color
encoding.
PNG objects might contain deviations from what is allowed in legal PNG
datastreams, if they were created by a
BASI
datastream and are
nonviewable.
In the case of a PNG object, the object also carries data from other
known PNG chunks that are present. This means that the decoder must
store sufficient information to make it possible to restore exactly the
original decoded and unfiltered pixel samples as they existed prior
to any gamma correction (but not the original compressed datastream
or line-by-line filter selections and "zlib" compression flags),
and data from the
IHDR
and
PLTE
chunks and any
additional recognized PNG chunks such as
gAMA
cHRM
and
tRNS
that the application plans to use. The sample depth,
color type, filter method, compression method, and interlacing method
of the image must be retained, and if the object has
been modified by a Delta-PNG, the
"pixel sample depth"
and
"alpha sample depth" must also be retained for use in decoding
subsequent Delta-PNG datastreams.
In the case of a JNG image, the object also carries data from other
known JNG chunks that are present. This means that the decoder must
store sufficient information to make it possible to restore exactly the
original JPEG datastream and decoded alpha channel as
they existed in the original JNG file, and data from the
JHDR
chunk and any additional recognized JNG chunks such as
gAMA
and
cHRM
that the application plans to use. As with PNG objects,
when the object has been modified by a Delta-PNG, the
"alpha sample depth" must also be retained for use in decoding
subsequent Delta-PNG datastreams. The "alpha compression method"
must be retained as well.
A decoder that recreates PNG or JNG files from a series of PNG,
JNG, and Delta-PNG datastreams will also have to store the contents of
any unknown chunks that it finds, in case they turn out to be safe to
copy (see
DROP (Paragraph 6.1.10)
DBYK (Paragraph 6.1.11)
and
ORDR (Paragraph 6.1.12)
below).
An abstract image. An abstract image can be stored by the decoder
in any form
that is convenient, such as an X Window System "pixmap", even
though that form might not have sufficient resolution for exact,
lossless conversion.
In the case of a PNG image, the pixels could be stored after the gamma
and chromaticity corrections have been made, and the sample depth could
be the same as the display hardware, even though it is smaller than
the original sample depth. Similarly, a JNG image could be stored in
the same form, after the pixels have been decoded, converted to RGB
form, and gamma-corrected. It is always safe, however, to store an
abstract image as though it were concrete, if decoders do not wish to
take advantage of the distinction between abstract and concrete objects.
Frozen or not frozen
All object buffers are initially "not frozen". Any object
buffers in existence when the
chunk is encountered
become "frozen".
Decoders do not actually have to store this flag except as a sanity
check, because they can depend on the fact that a "frozen" object
buffer will always have at least one "frozen" object
whose "buffer pointer" points to it.
A reference count
When an object buffer is first created, its reference count is set to
1.
When a partial clone is made of an object via the
CLON
chunk, the reference count for the object buffer is incremented, and no
new object buffer is created.
When an object is discarded and it points to an object
buffer that has a nonzero reference count, that reference count
is decremented and the object buffer is also
discarded if the resulting reference count is zero.
3.4. Object 0
Object 0 is a special object that has a set of object attributes
that control its location, clipping, and visibility properties, and
also has a set of magnification factors and methods, but does not have
an object buffer. The object attributes and magnification data,
which can be modified by the
DEFI
MOVE,
CLIP,
and
MAGN
chunks, are applied to subsequent
embedded objects whose
object_id
is zero. The pixel data for
object 0 is available only for on-the-fly display and not available for later use.
If at the end of any segment the attribute values or magnification data
are different from the default/saved values, they become undefined when
SEEK
chunk appears.
4. MNG Chunks
This chapter describes chunks that can appear at the top level of a
MNG datastream.
Unless otherwise specified in the Delta-PNG chapter of
this specification, they need not be recognized within a Delta-PNG datastream.
Chunk structure (length, name, data, CRC) and the chunk-naming system
are identical to those defined in the PNG specification
PNG
As in PNG, all
integers that require more than one byte must be in network byte order.
Unlike PNG, fields can be omitted from some MNG chunks with a default value
if omitted. This is permitted only when explicitly stated in the specification
for the particular chunk. If a field is omitted, all the subsequent fields in
the chunk must also be omitted and the chunk length must be shortened
accordingly.
4.1. Critical MNG control chunks
This section describes critical MNG control chunks that
MNG-compliant
decoders must recognize and process. "Processing" a
chunk sometimes can consist of simply recognizing it and ignoring it. Some
chunks have been declared to be critical only to prevent them from being
relocated by MNG editors.
4.1.1.
MHDR
MNG datastream header
The
MHDR
chunk is always first in all MNG datastreams except
for those that consist of a single PNG or JNG datastream with a PNG or
JNG signature.
The
MHDR
chunk contains 28 bytes, none of which can be omitted:
Frame_width: 4 bytes (unsigned integer).
Frame_height: 4 bytes (unsigned integer).
Ticks_per_second: 4 bytes (unsigned integer).
Nominal_layer_count: 4 bytes (unsigned integer).
Nominal_frame_count: 4 bytes (unsigned integer).
Nominal_play_time: 4 bytes (unsigned integer).
Simplicity_profile: 4 bytes:(unsigned integer).
bit 0: Profile Validity
0: Absence of any features is unspecified.
All other bits of the simplicity profile
must be zero (i.e, all other even numbers
are invalid).
1: Absence of certain features is specified by
the remaining bits of the simplicity profile.
(must be 1 in MNG-LC and MNG-VLC
datastreams)
bit 1: Simple MNG features
0: Simple MNG features are absent.
1: Simple MNG features may be present.
(must be 0 in MNG-VLC datastreams)
bit 2: Complex MNG features
0: Complex MNG features are absent.
1: Complex MNG features may be present.
(must be 0 in MNG-LC and MNG-VLC datastreams)
bit 3: Internal transparency
0: Transparency is absent or can be ignored.
All images in the datastream are opaque or
can be rendered as opaque without affecting
the final appearance of any frame.
1: Transparency may be present.
bit 4: JNG
0: JNG and JDAA are absent.
1: JNG or JDAA may be present.
(must be 0 in MNG-LC and MNG-VLC
datastreams)
bit 5: Delta-PNG
0: Delta-PNG is absent.
1: Delta-PNG may be present.
(must be 0 in MNG-LC and MNG-VLC datastreams)
bit 6: Validity flag for bits 7, 8, and 9
0: The absence of background transparency,
semitransparency, and stored object buffers
is unspecified; bits 7, 8, and 9 have no
meaning and must be 0.
1: The absence or possible presence of
background transparency is expressed by
bit 7, of semitransparency by bit 8, and of
stored object buffers by bit 9.
bit 7: Background transparency
0: Background transparency is absent (i.e., the
first layer fills the entire MNG frame with
opaque pixels).
1: Background transparency may be present.
bit 8: Semi-transparency
0: Semitransparency (i.e., an image with an
alpha channel that has values that are
neither 0 nor the maximum value) is absent.
1: Semitransparency may be present.
If bit 3 is zero this field has no meaning.
bit 9: Stored object buffers
0: Object buffers need not be stored.
1: Object buffers must be stored.
(must be 0 in MNG-LC and MNG-VLC
datastreams)
If bit 2 is zero, this field has no meaning.
bits 10-15: Reserved bits
Reserved for public expansion. Must be zero in
this version.
bits 16-30: Private bits
Available for private or experimental expansion.
Undefined in this version and can be ignored.
bit 31: Reserved bit. Must be zero.
Decoders can ignore the "informative"
nominal_frame_count
nominal_layer_count
nominal_play_time
, and
simplicity_profile
fields.
The
frame_width
and
frame_height
fields
give the intended display size (measured in
pixels) and provide
default clipping boundaries
(see Recommendations for encoders,
below
).
It is strongly recommended that these be set to zero if
the MNG datastream contains no visible images.
The
ticks_per_second
field gives the
unit used by the FRAM chunk to specify interframe delay and timeout.
In MNG-VLC datastreams, it gives the framing rate.
It must be nonzero if the datastream contains a sequence of images.
When the datastream contains exactly one frame,
this field should be set to zero.
When this field is zero, the length of a tick is infinite, and
decoders will ignore any
attempt to define interframe delay, timeout, or any other variable that
depends on the length of a tick. If the frames are intended to be
displayed one at a time under user control, such as a slide show or
a multi-page FAX, the tick length can be set to any positive number
and a
FRAM
chunk can be used to set an infinite interframe
delay and a zero timeout. Unless the user intervenes, viewers will only
display the first frame in the datastream.
When
ticks_per_second
is nonzero,
and there is no other
information available about interframe delay,
viewers should display the sequence of frames
at the rate of one frame per tick.
If the frame count field contains a zero, the frame
count is unspecified. If it is nonzero, it contains the number
of frames that would be displayed, ignoring the
fPRI
chunks and the
TERM
chunk. If the frame count is greater
than
31
-1
encoders should write
31
-1
, representing an infinite
frame count.
In MNG-VLC datastreams, the frame count is the same as the number of
embedded images in the datastream (or one, the background layer, if there are
no embedded images).
If the
nominal_layer_count
field contains a zero, the layer
count is unspecified. If it is nonzero, it contains the number of
layers (including
all background layers)
in the datastream, ignoring any effects of the
fPRI
chunks and the
TERM
chunk.
If the layer count is greater than
31
-1
, encoders
should
write
31
-1
, representing an infinite layer count.
In MNG-VLC datastreams, the layer count is the number of embedded images,
plus one (for the background layer).
If the
nominal_play_time
field contains a zero, the
nominal play time is unspecified. Otherwise, it gives the play time,
in ticks, when the file is displayed ignoring the
fPRI
chunks and the
TERM
chunk.
Authors who write this field should choose a
value of
ticks_per_second
that will allow the nominal play time
to be expressed in a four-bit integer. If the nominal play time is greater
than
31
-1
ticks, encoders should write
31
-1
representing an infinite nominal play time.
In MNG-VLC datastreams, the nominal play time is the same as the frame count,
except when the
ticks_per_second
field is zero, in which case the
nominal play time is also zero.
When bit 0 of the
simplicity_profile
field is zero, the
simplicity (or complexity) of the MNG datastream is unspecified,
and
all
bits of the simplicity profile must be zero.
The simplicity profile must be nonzero in
MNG-LC and MNG-VLC
datastreams.
If the simplicity profile is nonzero, it can be regarded
as a 32-bit profile, with bit 0 (the least significant bit) being a
"profile-validity" flag, bit 1 being a "simple MNG"
flag, bit 2 being a "complex MNG" flag, bits 3, 7, and 8 being
"transparency" flags, bit 4 being a "JNG" flag,
bit 5 being a "Delta-PNG" flag, and bit 9 being a
"stored object buffers" flag.
Bit 6 is a "validity" flag
for bits 7, 8, and 9, which were added at version 0.98 of this specification.
These three flags mean nothing if bit 6 is zero.
If a bit is zero, the corresponding feature is guaranteed
to be absent or if it is present there is no effect on the appearance of
any frame if the feature is ignored. If a bit is one, the
corresponding feature may be present in the MNG datastream.
Bits 10 through 15 of the simplicity profile are reserved for future
MNG versions, and must be zero in this version.
Bits 16 through 30 are available for private test or experimental versions.
The most significant bit (bit 31) must be zero.
When bit 1 is zero ("simple" MNG features are absent), the
datastream does not contain the
DEFI
FRAM
MAGN
or global
PLTE
and
tRNS
chunks, and filter method 64 is
not used in any embedded PNG datastream.
When bit 2 is zero, the datastream does not contain any "complex MNG
features". These are the
BASI
CLON
DHDR/IEND
PAST
DISC
MOVE
CLIP
, and
SHOW
chunks, or any chunk in a future version of this specification
that defines or uses stored objects. If
the
DEFI
chunk is present, it only defines object 0.
If the
BACK
chunk is present, it does not define
a background image.
If the
LOOP
chunk is present, it has
iteration_min=1
A MNG with a "complex MNG feature" (which has a simplicity
profile that has bit 2 set to 1) may contain
at least one of these chunks. A simple
decoder can display "simple" MNGs (which have a simplicity
profile with bit 2 set to 0) without having to store any
objects or dealing with the
SAVE/SEEK
mechanism, and it can ignore
the
LOOP
and
ENDL
chunks and execute all loops
exactly once.
"Transparency is absent or can be ignored" means that either the
MNG or
PNG
tRNS
chunk is not present and no PNG or JNG image has an alpha channel, or if
they are present they have no effect on the final appearance of any frame
and can be ignored
(e.g., if the only transparency in a MNG datastream
appears in a thumbnail that is never displayed in a frame, or is in some
pixels that are overlaid by opaque pixels before being displayed, the
transparency bit should be set to zero).
"Semitransparency is absent" means that if the
MNG or
PNG
tRNS
chunk is present or if any PNG or JNG image has an alpha channel, they only
contain the values 0 and the maximum (opaque) value. It also means that
the
JDAA
chunk is not present.
The "semitransparency"
flag means nothing and must be 0 if bit 3 is 0 or bit 6 is 0.
"Background transparency is absent" means
that the first layer of every segment fills the entire frame with
opaque pixels, and that
nothing following the first layer causes any frame to become transparent.
Whatever is behind the first layer does not show through.
When "Background transparency" is present, the application
is responsible for supplying a background color or image against which
the MNG background layer is composited, and if the MNG is being displayed
against a changing scene, the application should
refresh the entire MNG frame against a new copy of the background layer
whenever the application's background scene changes.
The "background transparency"
flag means nothing and must be 0 if bit 6 is 0.
Note that bit 3 does not
make any promises about background transparency.
The "stored object buffers" flag
is only useful when bit 2
is nonzero (i.e., "complex MNG features" are present). This
flag promises that even though such features are present, no chunk will
ever use the information in an existing object buffer; therefore it is not
necessary to store an object buffer for any object. A set of object attributes
is necessary for each object, however. Therefore, the
MOVE
CLIP
DISC
, deterministic
LOOP
partial
CLON
, and immediately-displayed
BASI
chunk
are permissible. The "stored object buffers" flag means nothing
if bit 2 is 0 or bit 6 is 0.
A MNG-LC (i.e., a "low-complexity MNG") datastream must have
a simplicity profile with bit 0 equal to 1
and all other bits except possibly for bits 1, 3, 6, 7, and 8
("simple MNG" MNG features and transparency) equal to zero.
If bit 4 (JNG) is 1, the datastream is a "MNG-LC that might contain
a JNG" datastream carrying an image or an alpha channel.
MNG-LC decoders are allowed to reject such datastreams unless they
have been enhanced with JNG capability.
A MNG-VLC (i.e., a "very low-complexity MNG") datastream must
have a simplicity profile with
bit 0 equal to 1 and all other bits except possibly for bits 3, 6, 7, and 8
(transparency)
equal to 0. If bit 4 (JNG) is 1, the datastream is a "MNG-VLC with
JNG" datastream. It might contain a
JNG datastream carrying an image or an alpha channel.
MNG-VLC decoders are allowed to reject such datastreams unless
they have been enhanced with JNG capability.
Encoders that write a nonzero simplicity profile should endeavor to
be accurate, so that decoders that process it will not unnecessarily
reject datastreams or avoid possible optimizations. For example, the
simplicity profile 351 (0x15f)
indicates that JNG, critical transparency, semitransparency,
and at least one "complex"
MNG feature are all present, but Delta-PNG, stored object buffers, and
background transparency are not. This example would not qualify as a
MNG-LC or a MNG-VLC
datastream because a "complex" MNG feature might be present.
If the simplicity profile
promises that certain features are absent, but they are actually present in
the MNG datastream, the datastream is invalid.
4.1.2.
MEND
End of MNG datastream
The
MEND
chunk's data length is zero. It signifies the end
of a MNG datastream.
4.1.3.
LOOP, ENDL
Define a loop
The
LOOP
chunk
provides a "shorthand" notation that can be
used to avoid having to repeat identical chunks in a MNG datastream.
The
LOOP
chunk
can be ignored by
MNG-LC and MNG-VLC
decoders, along with the
ENDL
chunk.
Its contents are the first two or more of the following fields. If
any field is omitted, all subsequent fields must also be omitted:
Nest_level: 1 byte (unsigned integer).
Iteration_count: 4 bytes (unsigned integer),
range [0..2^31-1].
Termination_condition:
1 byte (unsigned integer).
Must be omitted if termination_condition=0, which
means Deterministic, not cacheable, or if
iteration_count=0.
1: Decoder discretion, not cacheable.
2: User discretion, not cacheable.
3: External signal, not cacheable.
4: Deterministic, cacheable.
5: Decoder discretion, cacheable.
6: User discretion, cacheable.
7: External signal, cacheable.
Iteration_min: 4 bytes(unsigned integer). Must be present if
termination_condition is 3 or 7. If omitted, the
default value is 1.
Iteration_max: 4 bytes (unsigned integer). Must be present if
termination_condition is 3 or 7; must be omitted if
iteration_min is omitted; if omitted, the default
value is infinity.
Signal_number: 4 bytes (unsigned integer). Must be present if
termination_condition is 3 or 7. Must not be present
otherwise.
Additional
signal_number: 4 bytes. May be present only if termination_condition
is 3 or 7.
...etc...
Decoders must treat the chunks enclosed in a loop exactly as if they
had been repeatedly spelled out. Therefore, during the first iteration
of the loop, the parent objects for any Delta-PNG datastreams in the
loop are the images in existence prior to entering the
LOOP
chunk, but in subsequent iterations these parent objects might have been
modified. The
termination_condition
field can be used to
inform decoders that it is safe to change the number of loop iterations.
Simple decoders can ignore all fields except for the
iteration_count
When the
LOOP
chunk is present, an
ENDL
chunk
with the same
nest_level
must be present later in the MNG
datastream. Loops can be nested. Each inner loop must have a higher
value of
nest_level
than the loop that encloses it, though
not necessarily exactly one greater.
The termination condition specifies how the actual number of
iterations is determined. It is very similar to the termination
condition field of the
FRAM
chunk, and can take the same
values:
Deterministic
This is the default behavior, when the
termination_condition
field
is omitted or has a value that is unrecognized by the decoder.
The loop terminates after exactly the number of iterations
specified by the iteration count. This value must be used if altering
the number of repetitions would mess up the MNG datastream, but can be
used merely to preserve the author's intent.
Decoder-discretion
The number of iterations can be chosen by the decoder, and
must not be less than
iteration_min
nor more than
iteration_max
. If the decoder has no reason to choose its
own value, it should use the
iteration_count
. One example of a
decoder wishing to choose its own value is a real-time streaming decoder
hovering at a loop while waiting for its input buffer to fill to a
comfortable level.
User-discretion
The number of iterations should be chosen by the user (e.g., by
pressing the
iteration_min
and
iteration_max
limits. Some
decoders might not be able to interact with the user, and many decoders
will find that nested user-discretion loops present too great of a
user-interface challenge, so the
will probably usually degenerate into the
condition.
External-signal
The number of iterations must not be less than
iteration_min
nor more than
iteration_max
. The
exact number can be determined by the arrival of a signal whose number
matches one of the
signal_number
fields.
When the value of the
termination_condition
field is 4 or more, the
loop is guaranteed to be "cacheable", which means that every iteration
of the loop produces the same sequence of frames, and that all objects
and object buffers are left in the same condition at the end of each
iteration. Decoders can use this information to select a different
strategy for handling the loop, such as storing the composited frames
in a cache and replaying them rather than decoding them repeatedly.
The
iteration_min
and
iteration_max
can be omitted. If the condition is
values are not used. Otherwise,
defaults of 1 and
iteration_count
iteration_min
, and
iteration_max
can be any
non-negative integers or
iteration_min <= iteration_count <= iteration_max
Infinity is represented by 0x7fffffff. If all of the loops in
a MNG datastream have
iteration_min=1
, the datastream
can qualify as a "simple" MNG for the purpose of setting bits
1 and 2
of the "simplicity profile" to zero, unless there are other
reasons for setting them to one.
If
iteration_count
is zero,
the
termination_condition
, the subsequent fields must be
omitted, and the loop is done zero times.
Upon encountering a
LOOP
chunk whose
iteration_count
is zero,
decoders simply skip chunks until the matching
ENDL
chunk is
found, and resume processing with the chunk immediately following it.
The
signal_number
can be omitted only if the termination
condition is not
of
signal_number
fields.
Signal_number=0
is
reserved to represent any input from a keyboard or pointing device,
and 1-255 are reserved to represent the corresponding character code,
received from a keyboard or simulated keyboard, and values 256-1023 are
reserved for future definition by this specification.
An infinite or just overly long loop could give the appearance
of having locked up the machine. Therefore a decoder should always
provide a simple method for users to escape out of a loop or delay,
either by abandoning the MNG entirely or just proceeding to the next
SEEK
chunk (the
SEEK
chunk makes it safe for a
viewer to resume processing after it has jumped out of the interior
of a segment).
MNG editors that extract a series of PNG or JNG files from a MNG datastream
are expected to execute the loop only
iteration_min
times, when
the termination condition is not
The
ENDL
chunk ends a loop that begins
with the
LOOP
chunk. It contains a single one-byte field:
Nest_level: 1 byte (unsigned integer), range [0..255].
When the
ENDL
chunk is encountered, the loop
iteration count is decremented, if it is not already zero. If
the result is nonzero, processing resumes at the beginning of the loop.
Otherwise processing resumes with the chunk immediately following the
ENDL
chunk.
When the
ENDL
chunk is present, a
LOOP
chunk with
the same
nest_level
must be present earlier in the MNG
datastream. See
below
Loops must be properly
nested: if a
LOOP
chunk with higher
nest_level
appears inside a
LOOP/ENDL
pair, a matching
ENDL
chunk
must also appear to close it.
The
and
SEEK
chunks are not permitted
inside a
LOOP-ENDL
pair. To rerun an entire datastream that
includes these chunks, use the
TERM
chunk instead.
See
below (Paragraph 4.2.11)
4.2. Critical MNG image defining chunks
The chunks described in this section create
objects and initialize their object attributes, or change their object
attributes or the data in their object buffers. Some of them also
may cause images to be immediately displayed.
4.2.1.
DEFI
Define an object
The
DEFI
chunk sets the default set of object attributes
object_id
do_not_show
flag,
concrete_flag
, location, and clipping boundaries) for
any subsequent images that are defined with
IHDR-IEND
BASI-IEND
or
JHDR-IEND
datastreams.
If bit 1 of the
MHDR
simplicity profile is 0 and bit 0 is 1,
the
DEFI
chunk must not be present.
The
DEFI
chunk contains 2, 3, 4, 12, or 28 bytes. If any
field is omitted, all subsequent fields must also be omitted.
Object_id: 2 bytes (unsigned integer) identifier to be given to the
objects that follow the DEFI chunk. This field must be
zero in MNG-LC files.
Do_not_show: 1 byte (unsigned integer)
0: Make the objects potentially visible.
1: Make the objects not potentially visible.
Concrete_flag: 1 byte (unsigned integer)
0: Make the objects "abstract" (image cannot be the
source for a Delta-PNG)
1: Make the objects "concrete" (object can be the
source for a Delta-PNG).
MNG-LC decoders can ignore this flag.
X_location: 4 bytes (signed integer).
The X_location and Y_location fields can be omitted as
a pair.
Y_location: 4 bytes (signed integer).
Left_cb: 4 bytes (signed integer). Left clipping boundary. The
left_cb, right_cb, top_cb, and bottom_cb fields can be
omitted as a group.
Right_cb: 4 bytes (signed integer).
Top_cb: 4 bytes (signed integer).
Bottom_cb: 4 bytes (signed integer).
If the object number for an object is nonzero, subsequent chunks can
use this number to identify it.
When the object number for an object is zero, its object buffer
can be discarded immediately after it has been processed, and it can
be treated as an "abstract" image, regardless of the contents of the
concrete_flag
field.
Negative values are permitted for the X and Y location and clipping
boundaries. The left and top boundaries are inclusive, while the right and
bottom boundaries are exclusive.
The positive directions are downward and rightward from the
frame origin
(see Recommendations for encoders,
below
).
Multiple
IHDR-IEND
JHDR-IEND
, and
BASI-IEND
objects can follow a single
DEFI
chunk.
When
object_id
is nonzero, the
DEFI
chunk values
remain in effect until another
DEFI
chunk or a
SEEK
chunk appears, unless they are modified by
SHOW
MOVE
or
CLIP
chunks.
The
object_id
and
concrete_flag
can only be changed by using another
DEFI
chunk. If no
DEFI
chunk is in effect (either because there is none in
the datastream, or because a
DISC
or
SEEK
chunk has
caused it to be discarded),
the decoder must use the following default values:
Object_id = 0
Do_not_show = 0
Concrete_flag = 0
X location = 0
Y location = 0
Left_cb = 0
Right_cb = frame_width
Top_cb = 0
Bottom_cb = frame_height
The object attributes for all existing unfrozen objects except for object
0 become undefined when a
SEEK
chunk is encountered.
The object attributes for object 0 become undefined when a
SEEK
chunk is encountered, only if they have been reset to values
other than these defaults. It is the encoder's responsibility to
reset them explicitly to these values prior to the end of every segment
in which they have been changed, or to include a full
DEFI
chunk
prior to embedding object 0 in any segment.
These default values are also used to fill any fields that were omitted from
the
DEFI
chunk, when an object with the same
object_id
has not been previously defined or a
DISC
or
SEEK
chunk has
caused it to be discarded.
An set of object attributes is created or an existing one is modified when
the
DEFI
chunk appears, but an object buffer is neither created
nor discarded.
If
object_id
is an identifier that already exists when
DEFI
chunk appears, the set of object attributes (except for
the pointer to the object buffer) is immediately replaced. The
contents of the object buffer do not change, however, until and unless
an
IHDR
JHDR
BASI
, or
PAST
chunk is
encountered. When one of these chunks appears, all of the contents
of the object buffer previously associated with the identifier are discarded
and the new data is stored in the object buffer.
Note that if the object has partial clones, the object buffer of the clones
is naturally affected by the new data because it is shared, but the object
attributes sets of the clones are not affected.
4.2.2.
PLTE and tRNS
Global palette
The
PLTE
chunk has the same format as a PNG
PLTE
chunk. It provides a global palette that is inherited by PNG
datastreams that contain an empty
PLTE
chunk.
The
tRNS
chunk has the same format as a PNG
tRNS
chunk. It provides a global transparency array that is inherited along
with the global palette by PNG
datastreams that contain an empty
PLTE
chunk.
If a PNG datastream is present that does not contain an empty
PLTE
chunk, neither the global
PLTE
nor the
global
tRNS
data is inherited by that datastream.
If the global
PLTE
chunk is not present, each
indexed-color PNG in the datastream must supply its own
PLTE
(and
tRNS
, if it has transparency) chunks.
The global
PLTE
chunk is not permitted in MNG-VLC datastreams.
4.2.3.
IHDR
, PNG chunks,
IEND
A PNG (Portable Network Graphics) datastream.
See the PNG specification
PNG
and the Extensions to the PNG Specification document
PNG-EXT
for the format of the PNG chunks.
The
IHDR
and
IEND
chunks and any chunks between
them are written and decoded according to the PNG specification, except
as extended in this section. These extensions do not apply to standalone
PNG datastreams that have the PNG signature, but only to PNG datastreams
that are embedded in a MNG datastream that begins with a MNG signature.
Nor are they allowed in MNG-VLC datastreams.
An additional PNG filter method is defined:
64: Adaptive filtering with five basic types and intrapixel
differencing.
The intrapixel differencing transformation, which is a modification of
a method previously used in the LOCO image format
LOCO
is
S0 = Red - Green (when color_type is 2 or 6)
S1 = Green (when color_type is 2 or 6)
S2 = Blue - Green (when color_type is 2 or 6)
S3 = Alpha (when color_type is 6)
in which S0-S3 are the samples to be passed to the next stage of the
filtering procedure.
The transformation is done in integer arithmetic in sufficient
precision to hold intermediate results, and the result is calculated
modulo
sample_depth
Intrapixel differencing (subtracting the green
sample) is only done for color types 2 and 6, and only when the filter
method is 64. This filter method is not permitted in images with
color types other than 2 or 6.
Conceptually, the basic filtering is done after the
intrapixel differencing transformation has been done for all pixels
involved in the basic filter, although in practice the operations can
be combined.
To recover the samples, the transformation is undone after
undoing the basic filtering, by the inverse of the intrapixel differencing
transformation, which inverse is
Red = S0 + S1
Green = S1
Blue = S2 + S1
Alpha = S3
As in the forward transformation, the inverse
transformation is done
in integer arithmetic in sufficient precision to hold intermediate
results and the result calculated modulo
sample_depth
Applications that convert a MNG datastream to a series of PNG
datastreams must convert any PNG datastream with the additional filter
method 64 to a standard PNG datastream with a PNG filter method
(currently 0 is the only valid filter method).
The extra filter method can also be used in PNG datastreams that is
embedded in Delta-PNG and BASI datastreams.
It is suggested that encoders write a "nEED MNG-1.0" chunk if they use
this feature, for the benefit of pre-MNG-1.0 decoders.
Applications must not write MNG-VLC datastreams or independent PNG
datastreams (with either the .png or .mng file extension) with the new
filter method, until and unless it should become officially approved
for use in PNG datastreams.
If a global
PLTE
chunk appears in the top-level MNG
datastream, the PNG datastream can have an empty
PLTE
chunk
to direct that the global
PLTE
and
tRNS
data be used.
If an empty
PLTE
chunk is not present, the data is not inherited. MNG
applications that recreate PNG files must write the global
PLTE
chunk rather than the empty one in the output PNG file, along with the
global
tRNS
data if it is present. The global
tRNS
data can be subsequently overridden
by a
tRNS
chunk in the PNG datastream. It is an error
for the PNG datastream to contain an empty
PLTE
chunk when the
global
PLTE
chunk is not present or has been nullified.
If the PNG
sRGB
gAMA
iCCP
, or
cHRM
chunks appear in the top-level MNG datastream (and have
not been nullified), but none of them appear in the PNG datastream,
then the values are inherited from the top level as though the chunks
had actually appeared in the PNG datastream. Data from such chunks
appearing in the PNG datastream take precedence over the inherited
values. If any one of these chunks, or any chunk in a future
version of this specification that defines
the color space, appears in the PNG datastream, none of them is
inherited. MNG applications that recreate PNG files must write
these chunks, if they are inherited, in the output PNG files. If
the
sRGB
chunk is present in a MNG datastream, it need not
be accompanied in the MNG datastream by
gAMA
and
cHRM
chunks, despite the recommendation in the PNG
specification. Any MNG viewer that processes the
gAMA
chunk must
also recognize and process the
sRGB
chunk. It can treat
it as if it were a
gAMA
chunk containing the value .45455
and it can ignore its "intent" field. If the
sRGB
chunk
is present in the MNG datastream, editors that write PNG datastreams should
add the
gAMA
and
cHRM
chunks to the PNG datastream, even
though they are not present in the MNG datastream.
Note that the top-level color space chunks are used only to supply
missing color space information to subsequent embedded PNG or JNG datastreams.
They do not have any effect on already-decoded objects.
If the PNG
sPLT
chunk appears in the top-level MNG
datastream, it takes precedence over any
sPLT
chunk appearing
in the PNG datastream. MNG applications that recreate PNG files should
not copy top-level
sPLT
chunks to the output PNG files, because
a suggested palette for rendering a group of images is not necessarily
the best palette for rendering a single image.
The PNG
oFFs
and
pHYs
chunks and any chunks
in a future version of this specification
that attempt to set the pixel dimensions or the drawing location must
be ignored by MNG viewers and simply copied (according to the copying
rules) by MNG editors.
The PNG
gIFg
gIFt
, and
gIFx
chunks must
be ignored by viewers and must be copied according to the copying rules
by MNG editors.
If
do_not_show
is zero for the image when the
IHDR
chunk is encountered, a viewer can choose to display the image while
it is being decoded, perhaps taking advantage of the PNG interlacing
method, or to display it after decoding is complete.
If
object_id
is zero, there is no need to store the
pixel data after decoding it and perhaps displaying it.
If
concrete_flag=1
is 1 and
object_id
is nonzero,
the decoder must store the original pixel data losslessly, along
with data from other recognized PNG chunks, because it is possible
that a subsequent Delta-PNG datastream might want to modify it. If
concrete_flag
is zero, the decoder can store the pixel data in any
form that it chooses.
If the "stored object buffers" flag in the simplicity profile is
valid and zero, there is no need to store the pixel data and other chunk
data after decoding and perhaps displaying the image.
If an object already exists with the same
object_id
, the
contents of its object buffer are replaced with the new data.
4.2.4.
JHDR
, JNG chunks,
IEND
A JNG (JPEG Network Graphics) datastream.
See the JNG specification
below (Chapter 5)
for the format of the JNG datastream.
The
JHDR
and
IEND
chunks and any chunks between them
are written and decoded according to the JNG specification.
The remaining discussion in the previous paragraph about PNG
datastreams also applies to JNG datastreams.
MNG-LC and MNG-VLC
applications are not expected to process JNG
datastreams unless they have been enhanced with JNG capability.
4.2.5.
BASI
, PNG chunks,
IEND
The
BASI
chunk introduces a basis object that, while it
might be incomplete, can serve as a parent object to which a delta image
can be applied.
The first 13 bytes of the
BASI
chunk are identical to those
of the
IHDR
chunk. The next 8 bytes, which can be omitted,
provide sixteen-bit {red, green, blue, alpha} values that are used to fill
the entire basis object when the
IDAT
chunk is not present, and a
1-byte "viewable" flag can also be present.
Width: 4 bytes (unsigned integer).
Height: 4 bytes (unsigned integer).
Sample_depth: 1 byte (unsigned integer) 1, 2, 4, 8, or 16.
Color_type: 1 byte (unsigned integer) 0: Gray, 2: RGB, 3: indexed
color, 4: Gray-alpha, 6: RGBA
Compression_method: 1 byte (unsigned integer).
0: zlib with deflate
Filter_method: 1 byte (unsigned integer).
0: five basic filter types.
64: intrapixel differencing and five basic filter
types.
Interlace_method: 1 byte (unsigned integer).
0: none, 1: Adam7
Red_sample or
gray_sample: 2 bytes (unsigned integer).
Green_sample: 2 bytes (unsigned integer).
Blue_sample: 2 bytes (unsigned integer).
Alpha_sample: 2 bytes (unsigned integer).
Viewable: 1 byte (unsigned integer).
0: Basis object is not viewable.
1: Basis object is viewable.
The sample depth, color type, compression method, and
interlace method must be valid PNG types, and the width and height
must be within the valid range for PNG datastreams. The filter method
must be one of the filter methods allowed in PNG datastreams (currently
only 0) or the additional filter method (64) allowed in PNG
datastreams that are embedded in MNG datastreams.
The
alpha_sample
can be omitted if the
viewable
field is also omitted. If so, and the
color_type
is one
that requires alpha, the alpha value corresponding to an opaque
pixel will be used. If the color samples are omitted, zeroes will
be used. If the
viewable
field is omitted, the object is not
viewable.
The decoder is responsible for converting the color and
alpha samples to the appropriate format and sample depth for the
specified
color_type
The color and alpha samples are written as four sixteen-bit samples
regardless of the
color_type
and
sample_depth
When the
sample_depth
is less than sixteen, the least
significant bits are used and the remaining bits must be zero
filled.
When
color_type
is 0 or 4, the green and blue
samples must be present but must be ignored by decoders.
When
color_type
is 0 or 2,
only the values 0 and
sample_depth
should be written.
Any other alpha value must be interpreted as fully opaque.
When
color_type
is 3, the
decoder must generate a palette of length
sample_depth
whose first entry contains the given
{red_sample, green_sample,
blue_sample}
triple, and whose remaining entries are filled with
zeroes. It must also generate an alpha array whose first entry is the
given alpha sample and the rest are opaque
(i.e., if the alpha sample is not opaque, it creates a one-entry tRNS
chunk containing the least significant byte of the given alpha sample).
The
BASI
datastream contains PNG chunks, but is not
necessarily a PNG datastream. It can be incomplete or empty and it can
deviate in certain ways from the PNG specification. It can serve as a
parent object for a Delta-PNG datastream, which must supply the missing
data or correct the other deviations before the image is displayed. The
end of the datastream is denoted by an
IEND
chunk.
The
permitted deviations
from the PNG format
in a
BASI
datastream are:
The
IDAT
chunk can be omitted or there can be a single
empty
IDAT
chunk. If so, all of the pixels are filled with the
given color and alpha samples from the
BASI
chunk.
Multiple instances of some chunks can be present even though the PNG
specification allows only one. The subsequent Delta-PNG that uses this
as the parent object must select only one, through the
DBYK
or
similar mechanism. This deviation is only permitted when
the object is concrete and not viewable.
The
PLTE
chunk can be omitted or incomplete even when
color_type
is 3. If so, the subsequent Delta-PNG that uses this
as the parent object can supply a complete replacement
PLTE
chunk,
if the single-entry palette that is generated is not desired.
This deviation is only permitted when the object is concrete and not viewable.
The
BASI
chunk can be used to introduce such things as
a library of
iCCP
chunks from which one or another can be
selected for use with any single image, or it can be used to introduce a
simple blank or colored rectangle that will be immediately displayed or
into which other images will be pasted
by means of the
PAST
chunk.
BASI
chunk appearing in a MNG datastream receives its
object_id
, location, and potential visibility from the
preceding
DEFI
chunk, if one is present, or the default
values for
DEFI
, if one is not present. The
concrete_flag
can be either 0 (abstract) or 1 (concrete),
depending on whether the basis image is intended for subsequent use by
a Delta-PNG datastream or not. When it is abstract, it must also be
viewable. When it is viewable, the
resulting object, after the pixel samples are filled in, must be identical
to an object that would have been obtained by decoding a legal
PNG datastream. If
viewable
is 1 and
do_not_show
is 0, a viewer is expected to display
it immediately, as if it were decoding a PNG datastream.
If an object already exists with the same
object_id
, the
contents of its object buffer are replaced with the new data.
Top-level
gAMA
sRGB
cHRM
bKGD
sBIT
pHYs
iCCP
, and
sPLT
chunks
are inherited by a
BASI
datastream in the same manner as by a PNG
datastream.
No provision is made in this specification for storing a BASI
datastream as a standalone file. A
BASI
datastream will
normally be found as a component of a MNG datastream. Applications
that need to store a
BASI
datastream separately should use a
different file signature and filename extension. Better, they can wrap
it in a MNG datastream consisting of the MNG signature, the
MHDR
chunk, the
BASI
datastream, and the
MEND
chunk.
4.2.6.
CLON
Clone an object
Create a clone (a new copy) of an image, with a new
object_id
. The
CLON
chunk contains 4, 5, 6, 7, or
16 bytes. If a field is omitted, all subsequent fields must also be
omitted.
Source_id: 2 bytes (nonzero unsigned integer). Identifier of the
parent object to be cloned.
Clone_id: 2 bytes (nonzero unsigned integer). Identifier of the child
object that is created.
Clone_type: 1 byte (unsigned integer).
0: Full clone of the set of object attributes and the
object buffer.
1: Partial clone; only set of object attributes (the
location, clipping boundaries, and potential
visibility) are copied and a link is made to the
object buffer.
2: Renumber object (this is equivalent to
"CLON source_id clone_id 1
DISC source_id").
If this field is omitted, the clone_type defaults to zero
(full clone).
Do_not_show: 1 byte (unsigned integer).
0: Make the clone potentially visible and display it
immediately.
1: Make the clone not potentially visible.
When this field is omitted, the object retains the
potential visibility of the parent object.
Concrete_flag:
1 byte (unsigned integer).
0: Concrete_flag is the same as that of the parent
object.
1: Make the clone "abstract" (concrete_flag=0).
When this field is omitted, the object retains the
concrete flag of the parent object.
Loca_delta_type:
1 byte (unsigned integer)
0: Location data gives X_location and Y_location
directly.
1: New positions are determined by adding the location
data to the position of the parent object.
This field, together with the X_location and Y_location
fields, can be omitted as a group. When they are omitted,
the clone has the same location as the parent object.
X_location or delta_X_location:
4 bytes (signed integer).
Y_location or delta_Y_location:
4 bytes (signed integer).
The
source_id
must be an existing object identifier, and
the
clone_id
must not be an existing object identifier.
Negative values are permitted for the X and Y position. The positive
directions are downward and rightward from the frame origin.
The clone is initially identical to the parent object except for the
location and potential visibility. It has the same clipping boundaries
as the parent object. Subsequent
DHDR
SHOW
CLON
CLIP
MOVE
PAST
, and
DISC
chunks can use the
clone_id
to identify it. If
the parent object is not a viewable image, neither is the clone.
Subsequent chunks can modify, show, or discard a full clone or modify
its potential visibility, location and clipping boundaries without
affecting the parent object. They can also modify, show, or discard the
parent object or modify its set of object attributes without affecting the
clone.
The
concrete_flag
byte must be zero or omitted when the
clone_type
byte is nonzero.
If an object has partial clones, and the data in the object buffer
of a parent object or any of its partial clones is modified, the parent
object and all of its partial clones are changed. Decoders must take
care that when the parent object or any partial clone is discarded, the
object buffer is not discarded until the last remaining one of them
is discarded. Only the location, potential visibility, and clipping
boundaries can be changed independently for each partial clone.
If
viewable
is 1 and
do_not_show
is 0, the resulting image is displayed immediately.
4.2.7.
DHDR
, Delta-PNG chunks,
IEND
A Delta-PNG datastream.
See
The Delta-PNG Format (Chapter 6)
below, for the
format of the Delta-PNG datastream. Any chunks between
DHDR
and
IEND
are written and decoded according to the Delta-PNG
format. The
object_id
of the Delta-PNG
DHDR
chunk must point to an existing parent object. The resulting image
is immediately displayed if its
do_not_show
is 0. The parent
object must be concrete (i.e.,
concrete_flag
must be 1).
4.2.8.
PAST
Paste an image into another
Paste an image or images identified by
source_id
or part of it, into an existing abstract image identified by
destination_id
The
PAST
chunk contains a 2-byte
destination_id
and 9 bytes giving a "target location", plus one or more 30-byte source
data sequences.
Destination_id: 2 bytes (unsigned integer).
Target_delta_type:
1 byte (unsigned integer).
0: Target_x and target_y are given directly.
1: Target_x and target_y are deltas from their
previous values in a PAST chunk with the same
destination_id.
2: Target_x and target_y are deltas from their
previous values in the previous PAST chunk
regardless of its destination_id.
Target_x: 4 bytes (signed integer), measured rightward from the
left edge of the destination image.
Target_y: 4 bytes (signed integer), measured downward from the
top edge of the destination image.
Source_id: 2 bytes (unsigned nonzero integer). An image to be
pasted in.
Composition_mode:
1 byte (unsigned integer).
0: Composite over.
1: Replace.
2: Composite under.
Orientation: 1 byte (unsigned integer).
The source image is flipped to another orientation.
0: Same as source image.
2: Flipped left-right, then up-down.
4: Flipped left-right.
6: Flipped up-down.
8: Tiled with source image. The upper left corner of
the assembly is positioned according to the
prescribed offsets.
Offset_origin: 1 byte (unsigned integer).
0: Offsets are measured from the {0,0} pixel in the
destination image.
1: Offsets are measured from the {target_x,target_y}
pixel in the destination image.
X_offset: 4 bytes (signed integer).
Y_offset: 4 bytes (signed integer).
Boundary_origin: 1 byte (unsigned integer).
0: PAST clipping boundaries are measured from the
{0,0} pixel in the destination image.
1: PAST clipping boundaries are measured from the
{target_x,target_y} pixel in the destination image.
Left_past_cb: 4 bytes (signed integer).
Right_past_cb: 4 bytes (signed integer).
Top_past_cb: 4 bytes (signed integer).
Bottom_past_cb: 4 bytes (signed integer).
...etc...
The destination image must have the "abstract" property
(concrete_flag=0)
. When
destination_id=0
, the
resulting image is "write-only" and therefore
only "composite-over"
composition_mode=0
) operations are permitted.
The source images can be "abstract" or "concrete"
and have any
color_type
and
sample_depth
. They must
have the "viewable" property. The number of source images is
((chunk_length-11)/30)
The
x_offset
and
y_offset
distances and the
PAST
clipping boundaries are measured, in pixels, positive
rightward and
downward from either the
{0,0}
pixel of the destination image
or the
{target_x, target_y}
position in the destination
image. They do not necessarily have to fall within the destination
image. Only those pixels of the source image that fall within the
destination image and also within the specified clipping boundaries
will be copied into the destination image. The coordinate
system for offsets and clipping is with respect to the upper lefthand
corner of the destination image, which is not necessarily the same
coordinate system used by the
DEFI
MOVE
and
CLIP
chunks.
If the source image has been flipped or rotated,
X_offset
and
Y_offset
give the location of its new upper left hand corner.
When it is tiled, the offsets give the location of the upper left hand
corner of the upper left tile, and tiling is done to the right and down.
The
PAST
left and top clipping boundaries are inclusive, while the
right and bottom clipping boundaries are exclusive
(see Recommendations for encoders,
below
).
When
composition_mode=0
, any non-opaque pixels in the
source image are combined with those of the destination image. If
the destination pixel is also non-opaque, the resulting pixel will be
non-opaque.
When
composition_mode=1
, all pixels simply replace those
in the destination image. This mode can be used to make a transparent
hole in an opaque image.
When
composition_mode=2
, any non-opaque pixels in the
destination image are combined with those of the source image. If the
source pixel is also non-opaque, the resulting pixel will be non-opaque.
The order of composition is the same as the order that the
source_ids
appear in the list (but a decoder can do the
composition in any order it pleases, or all at once, provided that the
resulting destination image is the same as if it had actually performed
each composition in the specified order). Decoders must be careful when
the destination image equals the source image--the pixels to be drawn
are the ones that existed before the drawing operation began.
The clipping information from the
DEFI
MOVE
or
CLIP
chunks associated with the
destination_id
and the
source_ids
is not used in
the
PAST
operation (but if a decoder is simultaneously updating
and displaying the
destination_id
, the clipping boundaries
for the
destination_id
are used in the display
operation).
4.2.9.
MAGN
Magnify objects
This chunk provides mandatory magnification factors for existing objects
and/or for subsequent embedded images whose object id is 0.
The chunk contains 0 to 18 bytes. If any field is omitted, all subsequent
fields must also be omitted.
First_magnified_object_id:
2 bytes. If omitted, any previous MAGN chunk is
nullified.
Last_magnified_object_id:
2 bytes. If omitted, last object_id = first object_id.
X_method: 1 byte
0 or omitted: No magnification
1: Pixel replication of color and alpha samples.
2: Magnified intervals with linear interpolation of
color and alpha samples.
3: Magnified intervals with replication of color and
alpha samples from the closest pixel.
4: Magnified intervals with linear interpolation of
color samples and replication of alpha samples from
the closest pixel.
5: Magnified intervals with linear interpolation of
alpha samples and replication of color samples from
the closest pixel.
MX: 2 bytes. X magnification factor, range 1-65535. If
omitted, MX=1. Ignored if X_method is 0 and assumed to
be 1.
MY: 2 bytes. Y magnification factor. If omitted, MY=MX.
ML: 2 bytes. Left X magnification factor. If omitted, ML=MX.
MR: 2 bytes. Right X magnification factor. If omitted, MR=MX.
MT: 2 bytes. Top Y magnification factor. If omitted, MT=MY.
Ignored if Y_method is 0 and assumed to be 1.
MB: 2 bytes. Bottom Y magnification factor. If omitted,
MB=MY.
Y_method: 1 byte. If omitted, Y_method is the same as X_method.
The
MAGN
chunk causes the contents of the object buffers pointed to
by the specified range of objects to be immediately and irreversibly magnified.
The
first_magnified_object_id
can be zero. If so, any subsequent
embedded
objects whose
object_id
is 0 must be magnified immediately when they
appear in the datastream. Magnification factors and methods for object 0 are
updated by the appearance of a subsequent
MAGN
chunk whose
first_magnified_object_id
is 0. Magnification of object 0 is turned
off by the appearance of an empty
MAGN
chunk or by a
MAGN
chunk whose
first_magnified_object_id
is zero and
whose
X_method
and
Y_method
are zero, explicitly or by omission.
The magnification factor for object 0 becomes undefined when a
SEEK
chunk appears. Therefore, it is the encoder's responsibility either
to include a
MAGN
chunk that turns off magnification of object 0
prior to the end of any segment in which object 0 was magnified, or to
include a
MAGN
chunk for object 0 prior to the first embedded object 0
in every segment that contains an embedded object 0.
The
last_magnified_object_id
must be greater than or equal to the
first_magnified_object_id
. It
is not an error to include a nonexistent object or an existing
"frozen"
object in the range; decoders must do nothing to any such objects. If an
object is potentially visible and viewable, it is displayed immediately
after it is magnified. If any
object_id
is nonzero, the result
of magnifying that object is stored in place of its original object buffer
for later use.
If the
MAGN
chunk is present, all existing objects in the
specified range
must conceptually be magnified immediately in accordance with the given
magnification factors and methods. Decoders may wish to save the magnification
factors and delay the magnification until display time, or until the object
is used as the parent object of a Delta-PNG, to save memory. There is
nothing preventing this, provided that the end effect is the same as if the
magnification had been accomplished immediately. If object
0 is in the specified range, then any subsequent embedded objects with
object_id=0
must be magnified immediately when they appear in
the datastream.
When
X_method
is 0, all X magnification factors in
the
MAGN
chunk
are ignored and can be assumed to be 1.
When
X_method
is 1, X magnification is done by simple pixel
replication.
The leftmost pixel of each row is replicated
ML-1
times.
If the original
width is greater than 1, the rightmost pixel is replicated
MR-1
times.
If the original width is greater than 2, the original interior pixels are
replicated
MX-1
times. The magnified width W is
W = ML;
if (width > 1) W = W + MR;
if (width > 2) W = W + (width-2)*MX;
When
X_method
is 2, X magnification is done by linear interpolation between
pixels. If the original width of the image is greater than 1, the interval
between the leftmost pixel and the second pixel of each row is subdivided
into
ML
equal intervals by inserting
ML-1
pixels with
color and alpha values
that are obtained by linear interpolation. If the original width is 1, then
the pixel is simply magnified as if X method is 1. If the original width is
greater than 2, the rightmost interval is subdivided into MR equal intervals.
If the original width is greater than 3, each original interior interval is
subdivided into
MX
equal intervals. The magnified width W is
/* The orginal pixels: */
W = width;
/* Add the new pixels in the left interval: */
if (width > 1) W = W + ML-1;
/* Add the new pixels in the right interval: */
if (width > 2) W = W + MR-1;
/* Add the new interior pixels: */
if (width > 3) W = W + (width-3)*(MX-1);
When
X_method
is 3, intervals are subdivided as in X method 2,
and the
color and alpha values for the new pixels are obtained by replicating
the closest original pixel, with ties being broken by replicating the
pixel to the left. The magnified width is calculated in the same manner
as in X method 2.
When
X_method
is 4, the color samples are magnified as in
X method 2 and
the alpha samples are magnified as in X method 3.
When
X_method
is 5, the color samples are magnified as in
X method 3 and
the alpha samples are magnified as in X method 2.
When
Y_method
is 0, all Y magnification factors in
the
MAGN
chunk are ignored and can be assumed to be 1.
When
Y_method
is 1, Y magnification is done by simple pixel
replication.
The topmost pixel of each column is replicated
MT-1
times. If the
original
height is greater than 1, the bottom pixel is replicated
MB-1
times.
If the original height is greater than 2, the original interior pixels of each
column are replicated
MY-1
times. The magnified height H is
H = MT;
if (height > 1) H = H + MB;
if (height > 2) H = H + (height-2)*MY;
When
Y_method
is 2, Y magnification is done by linear interpolation between
pixels. If the original height of the image is greater than 1, the interval
between the topmost pixel and the second pixel of each column is subdivided
into MT equal intervals by inserting
MT-1
pixels with color and alpha
values
that are obtained by linear interpolation. If the original height is 1, then
the pixel is simply magnified as if Y method is 1. If the
original height is
greater than 2, the bottom interval is subdivided into MB equal intervals.
If the original height is greater than 3, each original interior interval is
subdivided into MY equal intervals. The magnified height H is
H = height;
if (height > 1) H = H + MT-1;
if (height > 2) H = H + MB-1;
if (height > 3) H = H + (height-3)*(MY-1);
When
Y_method
is 3, intervals are subdivided as in Y method 2,
and the
color and alpha values for the new pixels are obtained by replicating
the closest original pixel, with ties being broken by replicating the
pixel above. The magnified width is calculated in the same manner
as in Y method 2.
When
Y_method
is 4, the color samples are magnified as in
Y method 2 and
the alpha samples are magnified as in Y method 3.
When
Y_method
is 5, the color samples are magnified as in
Y method 3 and
the alpha samples are magnified as in Y method 2.
When the image being magnified is a concrete object, it must not be a
JNG or indexed-color PNG (the latter could be promoted to RGB or RGBA via
a Delta-PNG
PROM
chunk first). The result of the magnification is
also a concrete object. The Method 2 magnification is conceptually done first
in the vertical (Y) direction, the results rounded to the sample depth,
then in the horizontal (X) direction. Linear interpolation must be done
on the raw pixels, prior to any color correction,
using integer arithmetic, to ensure that the result is deterministic.
For each channel, the m-1 interpolated samples s[i] are obtained from the
two samples s0 and s1 by the following ISO C code or by any other method
that obtains the identical results:
if(s1 == s0)
for (i=1; i < m; i++)
s[i] = s0;
else
for (i=1; i < m; i++)
s[i] = ((2*i*(s1-s0)+m)/(m*2) + s0;
Signed arithmetic in a precision large enough to hold the intermediate
results must be used, and the final results must be modulo the sample depth.
When the image being magnified is an abstract object, which is always
true of object 0, interpolation can be done by any means that achieves a
visually similar but not necessarily identical result, such as rounding the
results to the sample depth later, using video hardware that is capable of
interpolation, or using floating point addition in the loop instead
of integer multiplication and division as in:
float delta = ((float)(s1-s0)/(float)m);
float sf= (float)s0;
for (i=1; i < m; i++) {
sf = sf+delta;
s[i]=(int)(sf+0.5);
If the abstract object being magnified is being stored in an indexed
representation, interpolation must be accomplished by a method that achieves
a similar result to that obtained by interpolating between RGB or RGBA
pixels.
Note that if an object and partial clones of it appear in the range
of objects to be magnified, the object buffer will be magnified repeatedly.
Because the
MAGN
chunk was added late in the development of
MNG-1.0, it is recommended that encoders place an empty
MAGN
chunk or
nEED MAGN
chunk early in the datastream, so that pre-MNG-1.0 applications that do not
recognize the
MAGN
chunk will encounter one quickly.
4.2.10.
DISC
Discard objects
The
DISC
chunk can be used to inform the decoder that it
can discard the object data associated with the associated object
identifiers. Whether the decoder actually discards the data or not, it
must not use it after encountering the
DISC
chunk.
The chunk contains a sequence of zero or more two-byte object
identifiers. The number of objects to be discarded is the chunk's data
length, divided by two.
Discard_id: 2 bytes (nonzero unsigned integer).
...etc...
If the
DISC
chunk is empty, all nonzero objects except those
preceding the
chunk (i.e., except for
the "frozen" objects)
can be discarded. If a
chunk has not been encountered, all
objects can be discarded. Note that each appearance of a
SEEK
chunk in the datastream implies an empty
DISC
chunk.
If the
DISC
chunk is not empty, the listed objects can be
discarded.
When an object is discarded, any location, potential visibility, and
clipping boundary data associated with it is also discarded.
It is not an error to include an
object_id
in the
discard_id
list, when no such object has been stored, or when
the object has already been discarded.
It is an error to name explicitly any "frozen" object in the
DISC
list.
When the object is a partial clone or is the source of a partial
clone that has not been discarded, only the set of object attributes
(location, potential visibility, clipping boundaries, etc.) can be
discarded. The data in the object buffer must be retained until the
last remaining partial clone is discarded.
4.2.11.
TERM
Termination action
The
TERM
chunk suggests how the end of the MNG datastream
should be handled, when a
MEND
chunk is found. It contains
either a single byte or ten bytes:
Termination_action: 1 byte (unsigned integer)
0: Show the last frame indefinitely.
1: Cease displaying anything.
2: Show the first frame after the TERM chunk.
If processing the fPRI chunk, use a "cost"
of 255.
3: Repeat the sequence starting immediately
after the TERM chunk and ending with the
MEND chunk.
Action_after_iterations: 1 byte
0: Show the last frame indefinitely after
iteration_max iterations have been done.
1: Cease displaying anything.
2: Show the first frame after the TERM chunk.
If processing the fPRI chunk, use a "cost"
of 255.
This and the subsequent fields must be present
if termination_action is 3, and must be omitted
otherwise.
Delay: 4 bytes (unsigned integer). Delay, in ticks,
before repeating the sequence.
Iteration_max: 4 bytes (unsigned integer). Maximum number of
times to execute the sequence. Infinity is
represented by 0x7fffffff.
The loop created by processing a
TERM
chunk must always be treated
by the decoder as if it were a cacheable
iteration_min=1
Applications must not depend on anything that has been drawn on the output
buffer or device during the previous iteration. Its contents become
undefined when the
TERM
loop restarts.
MNG editors that extract a series of PNG or JNG files from a MNG datastream
are expected to execute the
TERM
loop only once, rather than emitting
the files repeatedly.
The
TERM
chunk, if present, must appear either immediately
after the
MHDR
chunk or immediately prior to a
SEEK
chunk.
The
TERM
chunk is not considered to be a part of any
segment for the purpose of determining the copy-safe status of any
chunk.
Only one
TERM
chunk is permitted in a MNG datastream.
Simple viewers and single-frame viewers can ignore the
TERM
chunk. It has been made critical only so MNG editors will not
inadvertently relocate it.
4.3. Critical MNG image displaying chunks
The chunks in this section cause existing objects and embedded objects
to be displayed on the output device, and control their location, clipping,
and timing and the background against which they are displayed.
4.3.1.
BACK
Background
The
BACK
chunk suggests or mandates a background
color, image, or both
against which
transparent, clipped,
or less-than-full-frame images can be displayed. This information will be
used whenever the application subsequently needs to insert a background
layer, unless another
BACK
chunk provides new background information
before that happens.
The
BACK
chunk contains 6, 7, 9, or 10 bytes. If any field is
omitted, all subsequent fields must also be omitted.
Red_background: 2 bytes (unsigned integer).
Green_background: 2 bytes (unsigned integer).
Blue_background: 2 bytes (unsigned integer).
Mandatory_background:
1 byte (unsigned integer).
0: Background color and background image are
advisory. Applications can use them if they
choose to.
1: Background color is mandatory. Applications
must use it. Background image is advisory.
2: Background image is mandatory. Applications
must use it. Background color is advisory.
3: Background color and background image are both
mandatory. Applications must use them.
This byte can be omitted if the subsequent fields
are also omitted. If so, the background color is
advisory.
Background_image_id:
2 bytes (unsigned nonzero integer). Object_id of an
image that is to be used as the background layer or
part of it. If the image does not cover the area
defined by the layer clipping boundaries with opaque
pixels, the remainder of this area is filled with the
background color or application background and the
background image is composited against it. This
field can be omitted if the background_tiling byte is
also omitted; if so, no background image is defined,
and the background image_id from any previous BACK
chunk becomes undefined. This byte must be omitted
in MNG-LC and MNG-VLC datastreams, and when the
"stored object buffers" flag in the simplicity
profile is valid and is zero.
Background_tiling:
1 byte (unsigned integer).
0: Do not tile the background.
1: Tile the background with the background image.
This field can be omitted; if so, do not tile the
background. This byte must be omitted in MNG-LC and
MNG-VLC datastreams.
The first layer displayed by a viewer is always a
background layer that fills the entire frame.
The
BACK
chunk provides a background that the viewer can use
for this purpose (or must use, if it is mandatory). If it is
not "mandatory"
the viewer can choose another background if it wishes. If the
BACK
chunk is not present,
or if the background is not fully opaque or has been clipped to less than
full frame,
the viewer must provide
or complete
its own background layer for the first frame. Each layer
after the first must be composited over the layers that precede
it, until a
FRAM
chunk with framing mode 3 or 4 causes another
background layer to be generated.
Viewers are expected, however, to composite every foreground layer
against a fresh copy of the background, when the framing
mode given in the
FRAM
chunk is 3, and to composite the first
foreground layer of each subframe against a fresh copy of the background,
when the framing mode is 4. Also, when the framing mode is 3 or 4 and no
foreground layer appears between consecutive
FRAM
chunks,
a background layer alone is displayed as a separate frame.
The images and
the background are both clipped to the subframe boundaries given in the
FRAM
chunk. Anything outside these boundaries is inherited
from the previous subframe.
If the background layer is transparent and the subsequent foreground layers
do not cover the transparent area with opaque pixels, the application's
background becomes re-exposed in any uncovered pixels within the subframe
boundaries.
The background image (or tiled assembly) is also
clipped to its own boundaries and located like any other image,
and is only displayed if it is potentially visible. When
the background image is used for tiling, the upper left tile is located
according to the background image's location attributes and the entire
assembly is clipped according to its clipping attributes. Viewers might
actually follow some other procedure, but the final appearance of each
frame must be the same as if they had filled the area within the subframe
boundaries with the background color, then displayed the background
image, and then displayed the foreground image (or images), without
delay.
Note that any background layer, including the one that begins the
first frame of the datastream, must be inserted at the latest
possible moment, in case the background image is
replaced or is modified
by a Delta-PNG datastream or its location or clipping boundaries are
changed by a
MOVE
or
CLIP
chunk,
or in case a new
BACK
chunk appears,
before that moment.
It is an error to specify a
background_image_id
when the
"stored object buffers" flag in the simplicity profile is
valid and zero.
It is not an error to specify a
background_image_id
when such an image is not viewable and potentially visible or
does not yet exist or ceases to exist for some reason,
or to fail to specify one even when
the
mandatory_background
flag is 2 or 3.
Viewers must be prepared to fall back temporarily to using the background
color or application background in this event, and to resume using the
background image whenever a potentially visible viewable object with the
background_image_id
becomes available.
They also must be prepared for the contents, viewability, location, potential
visibility, and clipping boundaries of the background image to change, just
like any other object, if it has not been "frozen". The
background image is allowed to have transparency, subject to any promises
made in the simplicity profile.
The three
BACK
components are always written as though
for an RGBA PNG with 16-bit sample depth. For example, a mid-level
gray background could be specified with the RGB color samples
{1.09, 1.09, 1.09}.
The background color is interpreted in
the current color space as defined by any top-level
gAMA
cHRM
iCCP
sRGB
chunks that have appeared
prior to the
BACK
chunk in the MNG datastream. If no such
chunks appear, the color space is unknown.
The color space of the background image, if one is used, is determined
in the same manner as the color space of any other image.
The data from the
BACK
chunk takes effect the next time the
decoder needs to insert a background layer, and remains in effect until
another
BACK
chunk appears.
For the purpose of counting layers, when the background consists of
both a background color and a background image, these are considered to
generate a single layer and there is no delay
between displaying the background color and the background image.
Multiple instances of the
BACK
chunk are permitted in a MNG
datastream.
The
BACK
chunk can be omitted. If a background is needed
and the
BACK
chunk is omitted, then the viewer must supply its
own background. For the purpose of counting layers, such a viewer-supplied
background layer is counted the same as a background supplied by the
BACK
chunk.
In practice, most applications that use MNG as part of a
larger composition should ignore the
BACK
data if
mandatory_background=0
and the application already has
its own background definition. This will frequently be the case in
World Wide Web pages, to achieve nonrectangular transparent animations
displayed against the background of the page.
4.3.2.
FRAM
Frame definitions
The
FRAM
chunk provides information that a decoder needs for
generating frames and interframe delays. The
FRAM
parameters
govern how the decoder is to behave when it encounters a
FRAM
chunk, an embedded image, or a
SHOW
chunk.
The
FRAM
chunk also delimits subframes.
If bit 1 of the
MHDR
simplicity profile is 0 and bit 0 is 1,
the
FRAM
chunk must not be present.
An empty
FRAM
chunk is just a subframe delimiter. A
nonempty one is a subframe delimiter, and it also changes
FRAM
parameters, either for the upcoming subframe or until reset
("upcoming subframe" refers to the subframe immediately following the
FRAM
chunk). When the
FRAM
chunk is not empty, it contains a framing-mode byte, an
optional name string, a zero-byte separator, plus four 1-byte fields
plus a variable number of optional fields.
When the
FRAM
parameters are changed, the new parameters
affect the subframe that is about to be defined, not the one that is
being terminated by the
FRAM
chunk.
Framing_mode: 1 byte.
0: Do not change framing mode.
1: No background layer is generated, except for one
ahead of the very first foreground layer in the
datastream. The interframe delay is associated
with each foreground layer in the subframe.
2: No background layer is generated, except for one
ahead of the very first image in the datastream.
The interframe delay is associated only with the
final layer in the subframe. A zero interframe
delay is associated with the other layers in the
subframe.
3: A background layer is generated ahead of each
foreground layer. The interframe delay is
associated with each foreground layer, and a zero
delay is associated with each background layer.
4: The background layer is generated only ahead of
the first foreground layer in the subframe. The
interframe delay is associated only with the final
foreground layer in the subframe. A zero
interframe delay is associated with the background
layers, except when there is no foreground layer
in the subframe, in which case the interframe delay
is associated with the sole background layer.
Subframe_name: 0 or more bytes (Latin-1 Text). Can be omitted; if so,
the subframe is nameless.
Separator: 1 byte: (null). Must be omitted if the subsequent
fields are also omitted.
Change_interframe_delay:
1 byte.
0: No.
1: Yes, for the upcoming subframe only.
2: Yes, also reset default.
This field and all subsequent fields can be omitted as a
group if no frame parameters other than the framing mode
or the subframe name are changed.
Change_timeout_and_termination:
1 byte
0: No.
1: Deterministic, for the upcoming subframe only.
2: Deterministic, also reset default.
3: Decoder-discretion, for the upcoming subframe only.
4: Decoder-discretion, also reset default.
5: User-discretion, for the upcoming subframe only.
6: User-discretion, also reset default.
7: External-signal, for the upcoming subframe only.
8: External-signal, also reset default.
This field can be omitted only if the previous field is
also omitted.
Change_layer_clipping_boundaries:
1 byte.
0: No.
1: Yes, for the upcoming subframe only.
2: Yes, also reset default.
This field can be omitted only if the previous field is
also omitted.
Change_sync_id_list:
1 byte.
0: No.
1: Yes, for the upcoming subframe only.
2: Yes, also reset default list.
This field can be omitted only if the previous field is
also omitted.
Interframe_delay:
4 bytes (unsigned integer). This field must be omitted
if the change_interframe_delay field is zero or is
omitted. The range is [0..2^31-1] ticks.
Timeout: 4 bytes (unsigned integer). This field must be omitted
if the change_timeout_and_termination field is zero or
is omitted. The range is [0..2^31-1]. The value
2^31-1 (0x7fffffff) ticks represents an infinite
timeout period.
Layer_clipping_boundary_delta_type:
1 byte (unsigned integer).
0: Layer clipping boundary values are given directly.
1: Layer clipping boundaries are determined by adding
the FRAM data to the values from the previous
subframe.
This and the following four fields must be omitted if the
change_layer_clipping_boundaries field is zero or is
omitted.
Left_layer_cb or Delta_left_layer_cb:
4 bytes (signed integer).
Right_layer cb or Delta_right_layer_cb:
4 bytes (signed integer).
Top_layer_cb or Delta_top_layer_cb:
4 bytes (signed integer).
Bottom_layer_cb or Delta_bottom_layer_cb:
4 bytes (signed integer).
Sync_id: 4 bytes (unsigned integer). Must be omitted if
change_sync_id_list=0 and can be omitted if the new
list is empty; repeat until all sync_ids have been
listed. The range is [0..2^31-1].
Framing modes:
The
framing_mode
provides information to the decoder that it uses
whenever it is about to display an image, and when it is processing
the
next
FRAM
chunk.
Any of these events generates a layer, even
if no pixels are actually changed:
Decoding a
IHDR-IEND
sequence at the MNG level, when it
defines a potentially visible image.
Decoding a
JHDR-IEND
sequence at the MNG level, when it
defines a potentially visible image.
Decoding a
DHDR-IEND
sequence, when it defines a
potentially visible image.
Decoding a
BASI-IEND
sequence, when it defines a
potentially visible image.
Decoding a
CLON
chunk, when it defines a
potentially visible image.
Decoding a
PAST
chunk, when its destination is a
potentially visible image.
Decoding a
SHOW
chunk, when it directs that a potentially
visible image be displayed. When the
SHOW
chunk directs that
several images be displayed, each one in turn generates a separate layer
(or two layers, if the framing mode requires that a background layer be
inserted before each).
Decoding a
MAGN
chunk, when it directs that an existing
potentially visible image be magnified. When the
MAGN
chunk
directs that several images be magnified and displayed, each one in turn
generates a separate layer.
Also, decoding a
FRAM
chunk, when the current framing
mode requires a background layer (framing mode is 3 or 4) and none of
the above have already caused the background layer to be inserted
since the previous
FRAM
chunk. Such background layers must
be included in the
nominal_layer_count
field of the
MHDR
chunk.
When a decoder is ready to perform a display update, it must check
the framing mode, to decide whether it should
restore the background (framing modes 3 and 4) or not (framing modes 1 and 2),
and whether it needs to wait for the interframe delay to elapse before
continuing (framing modes 1 and 3) or not (framing modes 2 and 4).
When the interframe delay is zero, viewers are not required actually
to update the display but can continue to process the remainder of the
frame and composite the next image over the existing frame before displaying
anything. The final result must appear the same as if each image had been
displayed in turn with no delay.
Regardless of the framing mode, encoders must insert a
background layer, with a zero delay, ahead of the first image layer in the
datastream,
even when the
BACK
chunk is not present or has been clipped
to less than full-frame. This layer
must be included in the layer count but not in the frame count.
Also, viewers that jump to a segment must insert a
background layer, with a zero delay, ahead of the segment,
even when the
BACK
chunk is not present in the prologue segment,
if they jumped from the interior of a segment.
Such layers are
not
included in either the layer count
or the frame count.
Framing mode 1
When
framing_mode
is 1, the decoder must wait until the
interframe delay for the previous frame has elapsed before displaying
each image. Each foreground layer is a separate subframe and frame.
Framing mode 2
Framing mode 2 is the same as framing mode 1, except that the
interframe delay occurs between subframes delimited by
FRAM
chunks
rather than between individual layers.
All of the foreground layers
between consecutive
FRAM
chunks make up a single subframe.
In the usual case, the interframe delay is nonzero, and
multiple layers are present, so each
frame is a single subframe composed of several layers. When the interframe
delay is zero, the subframe is combined with subsequent subframes until one with
a nonzero interframe delay is encountered, to make up a single frame.
The decoder must wait until the interframe delay for the previous
frame has elapsed before displaying the frame.
When
framing_mode=2
, viewers are
expected to display all of the images in a frame at once, if possible, or as
fast as can be managed, without clearing the display or restoring the
background.
Framing mode 3
When
framing_mode=3
, a background layer is generated and
displayed immediately before each image layer is displayed. Otherwise,
framing mode 3 is identical to framing mode 1.
Each foreground layer together with its background layer make up a single
subframe and frame.
When the background layer is transparent or does not fill the clipping
boundaries of the image layer, the application
is responsible for supplying a background color or image against which
the image layer is composited, and if the MNG is being displayed
against a changing scene, the application should
refresh the entire MNG frame against a new copy of the background layer
whenever the application's background scene changes (see the
"background transparency" bit of the simplicity profile).
Framing mode 4
When
framing_mode=4
, the background layer is generated
and displayed immediately
before each frame, i.e., after each
FRAM
chunk, with no interframe
delay before each image. The decoder must wait until the
interframe delay for the previous frame has elapsed before displaying the
background layer.
Otherwise, framing mode 4 is identical to framing mode 2.
All of the foreground layers
between consecutive
FRAM
chunks, together with one background
layer, make up a single subframe.
A transparent or clipped background layer is handled as in framing mode 3.
The subframe name must conform to the same formatting rules as
those for a PNG
tEXt
keyword: It must consist only of printable
Latin-1 characters and must not have leading or trailing blanks, but
can have single embedded blanks. There must be at least one (unless
the subframe name is omitted) and no more than 79 characters in the
keyword. Keywords are case-sensitive. There is no null byte within
the keyword.
No specific use for the subframe name is specified in
this document, except that it can be included in the optional index
that can appear in the
chunk.
Applications can use this
field for such purposes as constructing an external list of subframe
in the datastream. The subframe name only applies to the upcoming
subframe; subsequent subframes are unnamed unless they also have their
own
frame_name
field. It is recommended that the same name
not appear in any other
FRAM
chunk or in any
SEEK
or
eXPI
chunk. Subframe names should not begin with the
case-insensitive
strings "CLOCK(", "FRAME(", or "FRAMES(",
which are reserved for use in URI queries and
fragments (see Uniform Resource Identifier
below
).
The interframe delay value is the desired minimum time to elapse from
the beginning of displaying one frame until the beginning of displaying
the next frame. When the interframe delay is nonzero, which will
probably be the usual case, layers are frames. When it is zero, a
frame consists of any number of consecutive subframes, until
a nonzero delay subframe is encountered and completed. Decoders are not
obligated or encouraged to display such subframes individually; they can
composite them offscreen and only display the complete frame.
There is no interframe delay before the first layer (the implicit
background layer) in the datastream nor after the final frame, regardless
of the framing mode.
The timeout field can be a number or
can be represented by 0x7fffffff. Under certain termination conditions,
the application can adjust the interframe delay, provided that it is
not greater than the sum of the specified interframe delay and
the timeout.
The termination condition given in the
change_timeout_and_termination
field specifies whether and
over what range the normal interframe delay can be lengthened or
shortened. It can take the following values:
deterministic
The frame endures no longer than the normal interframe delay. Even
though this is the default, a streaming encoder talking to a real-time
decoder might write a
FRAM
with a termination condition of
"deterministic" to force the display to be updated while the encoder
decides its next move.
decoder-discretion
If the interframe delay is nonzero, the decoder can shorten or lengthen
the duration of the frame, to any duration between the interframe delay
and the timeout. A streaming decoder could take the opportunity to
wait for its input buffer to fill to a comfortable level.
user-discretion
If the interframe delay is nonzero, the decoder should wait for
permission from the user (e.g., via a keypress) before proceeding, but
must wait no less than the smaller of the timeout and the interframe
delay nor no longer than the greater of the timeout and the interframe
delay. If the decoder cannot interact with the user, this condition
degenerates into "decoder-discretion".
external-signal
If the interframe delay is nonzero, the decoder should wait for
the arrival of a signal whose number matches a
sync_id
, but
must wait no less than the smaller of the timeout and the interframe
delay nor no longer than the greater of the timeout and the interframe
delay. If the decoder cannot receive signals, this condition degenerates
into "decoder-discretion".
The
sync_id
list can be omitted if the termination
condition is not "external-signal".
When the
sync_id
list is changed, the number of
sync_id
entries is determined by the remaining length of the
chunk data, divided by four. This number can be zero, which either
inactivates the existing
sync_id
list for one frame or
deletes it.
The initial values of the
FRAM
parameters are:
Framing mode = 1
Subframe name =
Interframe delay = 1
Left subframe boundary = 0
Right subframe boundary = frame_width
Top subframe boundary = 0
Bottom subframe boundary = frame_height
Termination = deterministic
Timeout = 0x7fffffff (infinite)
Sync id =
The layer clipping boundaries from
the
FRAM
chunk are only
used for clipping, not for placement.
The
DEFI
or
MOVE
chunk
can be used to specify the placement of each
image within the layer. The
DEFI
or
CLIP
chunk can be used to specify
clipping boundaries for each image.
Even when the left and top subframe
boundaries are nonzero, the image locations are measured with respect to
the {0,0} position in the display area.
The left and top subframe boundaries are inclusive, while the right and bottom
boundaries are exclusive.
If the layers
do not cover the entire area defined by the layer clipping
boundaries with opaque pixels, they are composited against
whatever already occupies the area, when the framing mode is 1 or 2.
When the framing mode is 3 or 4, they are composited against
the background defined by the
BACK
chunk, or against an application-defined background, if
the
BACK
chunk is not present or does not define a mandatory
background. The images, as well as the background, are clipped to the
layer clipping boundaries for the subframe. Any pixels outside the
layer clipping boundaries remain unchanged from the previous layer.
The
interframe_delay
field gives the duration of
display, which is the minimum time that must elapse from the
beginning of displaying one layer until the beginning of displaying
the next (unless the termination condition and timeout permit this
time to be shortened). It
is measured in "ticks" using the tick length determined from
ticks_per_second
defined in the
MHDR
chunk.
When the interframe delay is zero, it indicates that
the layer is to be combined with the subsequent layer or layers into a
single frame, until a nonzero interframe delay is specified or
the
MEND
chunk is reached.
A viewer does not actually have to follow the procedure of erasing
the screen, redisplaying the background, and recompositing the images
against it, but what is displayed when the frame is complete must be the
same as if it had. It is sufficient to redraw the parts of the display
that change from one frame to the next.
The
sync_id
list provides a point at which the processor
must wait for all pending processes to reach the synchronization
point having the same
sync_id
before resuming, perhaps
because of a need to synchronize a sound datastream (not defined
in this specification) with the display, to synchronize stereo
images, and the like. When the period defined by the sum of the
interframe_delay
and the
timeout
fields
elapses, processing can resume even though the processor has not
received an indication that other processes have reached the
synchronization point.
Note that the synchronization point does not occur immediately, but
at the end of the first frame that follows the
FRAM
chunk.
The identifier
sync_id=0
is reserved to represent
synchronization with a user input from a keyboard or pointing device.
The
sync_id
values 1-255 are reserved to represent the
corresponding ASCII letter, received from the keyboard (or a simulated
keyboard), and values 256-1023 are reserved for future definition
by this specification. If multiple channels (not defined in this
specification) are not present, viewers can ignore other values
appearing in the
sync_id
list.
Note that the rules for omitting the interframe delay, timeout, clipping
boundary, and sync id fields of the
FRAM
chunk are different from
the general rule stated in MNG Chunks,
above (Chapter 4)
These fields are either present in the chunk data or omitted from it according
to the contents of the corresponding "change" byte.
4.3.3.
MOVE
New image location
The
MOVE
chunk gives a new location of an existing object or
objects (replacing or incrementing the location given in the
DEFI
chunk).
The position is measured downward and
to the right of the frame origin, in pixels, where the named object or
group of objects is to be located.
The chunk's contents are:
First_object: 2 bytes (unsigned integer).
Last_object: 2 bytes (unsigned integer).
Location_delta_type:1 byte (unsigned integer).
0: MOVE data gives X_location and Y_location
directly.
1: New locations are determined by adding the MOVE
data to the location of the parent object.
X_location or delta_X_location:
4 bytes (signed integer).
Y_location or delta_Y_location:
4 bytes (signed integer).
The new location applies to a single object, if
first_object=last_object
, or to a group of consecutive
object_ids
, if they are different.
Last_object
must not be less than
first_object
. Negative values are
permitted for the X and Y location. The positive directions are
downward and rightward from the frame origin. The
MOVE
chunk
can specify an image placement that is partially or wholly outside the
display boundaries. In such cases, the resulting image must be clipped
to fit within its clipping boundaries, or not displayed at all if it
falls entirely outside its clipping boundaries. The clipping boundaries
are determined as described in the specification for the
CLIP
chunk
below (Paragraph 4.3.4)
The left and top boundaries are inclusive, while the right and bottom
boundaries are exclusive.
It is not an error for the
MOVE
chunk to name an object that
has not previously been defined. In such cases, nothing is done to the
nonexistent object. It is permitted to move "frozen" objects
provided that the encoder includes chunks to move them back to their
original positions prior to then end of the segment.
When an object is discarded, its set of object attributes, which includes
the
MOVE
data, is also discarded.
4.3.4.
CLIP
Object clipping boundaries
This chunk gives the new boundaries (replacing or incrementing those
from the
DEFI
chunk) to which an existing object or group of
objects must be clipped for display. It contains the following 21
bytes:
First_object: 2 bytes (unsigned integer).
Last_object: 2 bytes (unsigned integer).
Clip_delta_type: 1 byte (unsigned integer).
0: CLIP data gives boundary values
directly.
1: CLIP boundaries are determined by
adding the CLIP data to their
previous values for this object.
Left_cb or delta_left_cb: 4 bytes (signed integer).
Right_cb or delta_right_cb: 4 bytes (signed integer).
Top_cb or delta_top_cb: 4 bytes (signed integer).
Bottom_cb or delta_bottom_cb: 4 bytes (signed integer).
The new clipping boundaries apply to a single object, if
first_object=last_object
, or to a group of consecutive
objects, if they are different. the
last_object
must not be less
than
first_object
The clipping boundaries are expressed in pixels, measured rightward
and downward from the frame origin.
The left and top clipping boundaries are inclusive and the right and
bottom clipping boundaries are exclusive, i.e., the pixel located at
{x,y} is only displayed if the pixel falls within the physical limits of
the display hardware and all of the following are true:
0 <= x < frame_width (from the MHDR chunk)
0 <= y < frame_height
Left_lcb <= x < right_lcb (from the FRAM chunk)
Top_lcb <= y < bottom_lcb
Left_cb <= x < right_cb (from the CLIP chunk)
Top_cb <= y < bottom_cb
It is not an error for the
CLIP
chunk to name an object that
has not previously been defined. In such cases, nothing is done to the
nonexistent object. It is permitted to clip "frozen" objects
provided that another
CLIP
chunk resets them to their original
values prior to the end of the segment.
When an object is discarded, its set of object attributes, which includes
the
CLIP
data, is also discarded.
4.3.5.
SHOW
Show images
The
SHOW
chunk is used to change the potential visibility
of one or more previously-defined objects and to direct that they be
displayed. It contains 2, 4, or 5 bytes, or it can be empty. When any
field is omitted, all subsequent fields must also be omitted.
First_image: 2 bytes (nonzero unsigned integer).
Last_image: 2 bytes (nonzero unsigned integer). This field can be
omitted if the show_mode byte is also omitted. If so,
decoders must assume the default values, show_mode=0 and
last_image=first_image.
Show_mode: 1 byte (unsigned integer).
0: Make the images potentially visible and display them
(set do_not_show=0).
1: Make the images invisible (set do_not_show=1).
2: Do not change do_not_show flag; display those that are
potentially visible.
3: Mark images "potentially visible" (do_not_show=0), but
do not display them.
4: Toggle do_not_show flag; display any that are
potentially visible after toggling.
5: Toggle do_not_show flag, but do not display even if
potentially visible after toggling.
6: Step through the images in the given range, making the
next image potentially visible (set do_not_show=0) and
display it. Set do_not_show=1 for all other images in
the range. Jump to the beginning of the range when
reaching the end of the range. Perform one step for
each SHOW chunk (in reverse order
if last_image < first_image).
7. Make the next image in the range (cycle) potentially
visible (do_not_show=0), but do not display it. Set
do_not_show=1 for the rest of the images in the range.
This field can be omitted. If so, decoders must assume the
default, show_mode=0.
The decoder processes the objects (or images) named in the
SHOW
chunk in the order
first_image
through
last_image
, and resets the
do_not_show
flag for
each of the objects. If
show_mode
is even-valued, it also
displays the images if they are potentially visible and are viewable
images.
When the
SHOW
chunk is empty, the decoder displays
all existing potentially visible images, without changing their
do_not_show
status. The empty
SHOW
chunk is
equivalent to
SHOW 1 65535 2
If
last_image < first_image
the images are processed in
reverse order.
When
show_mode
is odd-valued, nothing is displayed
unless a subsequent
SHOW
chunk with an even-valued
show_mode
appears.
Interactions with the framing mode
When
show_mode
is even-valued, each visible image that
is displayed generates a separate layer, even
if it is offscreen and no pixels are actually displayed. In such cases,
the layer is totally transparent. When
show_mode
is odd, or
when no image is potentially visible and
show_mode
is 2, 4, or
empty, no layer is generated.
When
show_mode
is 1, 4, 5, 6, or 7, images can
be made invisible. This is not permitted when the framing mode is 2 or 4
in the
FRAM
chunk and the images have already appeared in the
frame, because simple viewers will have already drawn them and have no
way to make them invisible again without redrawing the entire frame.
When
show_mode
is 6 or 7, the decoder must make the
next image in the "cycle" potentially visible and, if
show_mode
is 6, generate a single layer. To do this,
it must examine
the
do_not_show
flag for each image in the range
first_image
through
last_image
, and make the next
one (the one with the next higher value of
image_id
that
exists and is "viewable") after the first visible one it finds
visible
and the rest invisible. When
first_image > last_image
the cycle is reversed, and the "next" image is the one with the next
lower value of
image_id
. In either case, if the first
visible one found was
last_id
, or none were visible, it must make
first_image
visible. These modes are useful for manipulating
a group of sequential images that represent different views of an
animated icon. See Example 8,
below (Chapter 18)
If no "viewable" object is in the specified range, an empty layer
must be generated.
When
show_mode
is 0, 2, 4, or 6, separate layers will be
generated, each containing an instance of one visible image at the
location specified by the
DEFI
CLON
, or
MOVE
chunk and clipped according to the boundaries specified by the
CLIP
and
FRAM
chunks. When the
MOVE
or
CLON
chunk is used in the delta form, which will frequently be
the case, each image must be displaced from its previous position by the
values given in the
MOVE
or
CLON
chunk.
Assuming a nonzero interframe delay, any of the following sequences
would cause the image identified by
object_id=6
in a
composite frame to blink:
LOOP 0 0 10
FRAM 4 # Show background
SHOW 1 10 # Show images 1 thru 10.
FRAM # Show background
SHOW 1 5 # Show images 1 thru 5.
SHOW 7 10 # Show images 7 thru 10.
ENDL
FRAM 4 # Show background
LOOP 0 0 10
SHOW 1 5 # Show images 1 thru 5.
SHOW 6 6 4 # Toggle potential visibility of image 6
SHOW 7 10 # and show it; show images 7 thru 10.
FRAM
ENDL
FRAM 4 # Show background
LOOP 0 0 10
SHOW 6 6 5 # Toggle potential visibility of image 6.
SHOW 1 10 2 # Show potentially visible images in 1
FRAM # through 10.
ENDL
It is not necessary to follow an
IHDR-IEND
JHDR-IEND
BASI-IEND
, or
DHDR-IEND
sequence or
PAST
chunk with a
SHOW
chunk to
display the resulting image, if it was already caused to appear by
do_not_show=0
in the
DEFI
chunk that introduced the
image. Similarly, the
CLON
chunk need not be followed by a
SHOW
chunk, if
do_not_show=0
in the
CLON
chunk.
It is not an error for the
SHOW
chunk to name
a nonviewable object or an object that
has not previously been defined. In such cases, nothing is done to the
nonexistent object.
It is permitted to change the potential visibility of "frozen"
objects provided that another
SHOW
chunk resets them to their original
values prior to the end of the segment.
4.4.
and
SEEK
chunks
The
chunk marks a point in the datastream at which
objects are "frozen" and other chunk information
is "saved". The
SEEK
chunk marks positions in the MNG datastream where a
restart is possible, and where the decoder must restore the "saved"
information, if they have jumped or skipped to a
SEEK
point from
the interior of a segment.
They only need to restore information that they will use, e.g., a viewer
that processes
gAMA
and global
PLTE
and
tRNS
but ignores
iCCP
and
sPLT
, need only restore the value of gamma
and the global
PLTE
and
tRNS
data from the prologue
segment but not the values of the
iCCP
and
sPLT
data.
Simple decoders that only read MNG datastreams sequentially can
safely ignore the
and
SEEK
chunks, although it
is recommended that, for efficient use of memory, they at least mark
existing objects as "frozen" when the
chunk is
processed and discard all "unfrozen" objects whenever
the
SEEK
or empty
DISC
chunk is processed.
4.4.1.
Save information
The
chunk marks a point in the datastream at which
objects are "frozen" and other
chunk information is "saved"; a decoder skipping
or jumping to a
SEEK
chunk from the interior of a segment
must restore the "saved"
chunk information if it has been redefined or discarded. In addition,
the
chunk can contain an optional index to the MNG
datastream.
The
chunk can be empty, or it can contain an index
consisting of the following:
Offset_size: 1 byte (unsigned integer).
4: Offsets and nominal start times are expressed as
32-bit integers.
8: Offsets and nominal start times are expressed as
64-bit integers.
plus zero or more of the following index entries:
Entry_type: 1 byte (unsigned integer).
0: Segment with nominal start time, nominal layer number,
and nominal frame number.
1: Segment.
2: Subframe.
3: Exported image.
Offset: 4 or 8 bytes (unsigned integer). Must be omitted if
entry_type > 1, set equal to zero if the offset is
unknown.
Nominal_start_time:
4 or 8 bytes: (unsigned integer). Start time of the segment,
measured in ticks from the beginning of the sequence,
assuming that all prior segments were played as intended on
an ideal player, ignoring any fPRI chunks. Must be omitted
if entry_type > 0.
Nominal_layer_number:
4 bytes (unsigned integer). Sequence number of the first
layer in the segment, assuming that all prior segments were
played as intended on an ideal player, ignoring any fPRI
chunks; the first layer of the first segment being layer 0.
Must be omitted if entry_type > 0.
Nominal_frame_number:
4 bytes (unsigned integer). Sequence number of the first
frame in the segment, assuming that all prior segments were
played as intended on an ideal player, ignoring any fPRI
chunks; the first frame of the first segment being frame 0.
Must be omitted if entry_type > 0.
Name: 1-79 bytes (Latin-1 text). Must be omitted for unnamed
segments. The contents of this field must be the same as
the name field in the corresponding SEEK, FRAM, or eXPI
chunk.
Separator: 1 byte (null) (must be omitted after the final entry).
The
chunk must be present when the
SEEK
chunk is present. It appears after the set of chunks that define
information that must be retained for the remainder of the datastream.
These chunks, collectively referred to as the prologue segment, are no
different from chunks in other segments. They can be chunks
that define objects, or they can be chunks that define other
information such as
gAMA
cHRM
, and
sPLT
. If any chunks appear
between the
chunk and the first
SEEK
chunk, these
chunks also form a part of the prologue segment, but their contents become
undefined when the
SEEK
chunk appears.
Only one instance of the
chunk is permitted in a MNG
datastream. It is not allowed anywhere after the first
SEEK
chunk.
It is not permitted, at any point beyond the
chunk, to
modify or discard any object that was defined ahead of the
chunk.
An object appearing ahead of the
chunk can be the
subject of a
CLON
chunk. If the clone is a partial clone,
modifying it is not permitted, because this would also modify the object
buffer that the original object points to.
A chunk like
gAMA
that overwrites a single current value is
permitted after the
chunk, even if the chunk has appeared
ahead of the
chunk. Decoders are responsible for saving a
copy of the chunk data (in any convenient form) when the
chunk is encountered and restoring it when skipping or jumping to a
SEEK
chunk from the interior of a segment. If no instance of
the chunk appeared ahead of the
chunk, the decoder must restore the chunk data to its
original "unknown" condition when it skips or jumps to
SEEK
chunk from the interior of a segment.
It is the
encoder's
responsibility, if it changes or
discards any "saved" data, to restore it to
its "saved" condition (or
to nullify it, if it was unknown) prior to the end of the segment. This
makes it safe for simple decoders to ignore the
SAVE/SEEK
mechanism.
Known chunks in this category include
DEFI
FRAM
BACK
PLTE
cHRM
tRNS
fPRI
gAMA
iCCP
bKGD
sBIT
pHYg
pHYs
, and
sRGB
In addition, it is the responsibility of the encoder to include chunks
that restore the
potential visibility, location, and clipping boundaries of any
"frozen" objects to their "saved" condition.
In the case of chunks like
sPLT
that can occur multiple
times, with different "purpose" fields, additional instances of the
chunk are permitted after the
chunk, but not with the
same keyword as any instances that occurred ahead of the
chunk. The decoder is required to forget such additional instances when
it skips or jumps to a
SEEK
chunk from the interior of a segment,
but it must retain those instances that were defined prior to the
chunk. Encoders are required to nullify such additional instances prior to the
end of the segment. Known chunks in this category include only
sPLT
If an entry for a segment (entry type 0 or 1) appears in the optional
index, there must also be an entry for every segment, whether named or
not, except for the prologue segment, that precedes it.
All entries must appear in the index in the same order that they appear in
the MNG datastream.
There must never be a segment entry (type 0 or 1) for the prologue segment, but
there can be entries for named images or subframes in the prologue, placed
ahead of the first segment entry.
Only named images or subframes are permitted, and it is not
an error to omit any or all named images or subframes. Nor is it an error to
omit a contiguous set of segments at the end of the datastream from the index.
Offsets are calculated from the first byte of the MNG 8-byte
signature, which has offset=0. This is true even if the MNG datastream
happens to be embedded in some other file and the signature bytes are
not actually present.
Applications with direct access to the datastream can use the index
to find segments, subframes, and exported images quickly. After
processing the prologue segment, they can jump directly to any segment
and then process the remaining datastream until the desired subframe,
image, or time is found. Applications that have only streaming access
to the datastream can still use the index to decide whether to decode
the chunks in a segment or to skip over them.
Only one instance of the
chunk is permitted in a MNG
datastream. If the
SEEK
chunk is present, the
chunk must be present, prior to the first
SEEK
chunk. The
only chunks not allowed ahead of the
chunk are the
SEEK
chunk and the
MEND
chunk. The
chunk must not appear inside a
LOOP-ENDL
pair.
4.4.2.
SEEK
Seek point
The
SEEK
chunk marks positions ("seek points")
in the MNG
datastream where a restart is possible, and where the decoder must restore
certain information to the condition that existed when the
chunk
was processed, if it has skipped or jumped to the
SEEK
chunk from
the interior of a segment.
The
SEEK
chunk can be empty, or it can contain a segment name.
Segment_name: 1-79 bytes (Latin-1 string).
The segment name is optional. It must follow the format of a
tEXt
keyword: It must consist only of printable Latin-1
characters and must not have leading or trailing blanks, but can have
single embedded blanks. There must be at least one and no more than 79
characters in the keyword. There is no null byte terminator within the
segment name, nor is there a separate null byte terminator. Segment
names are case-sensitive. Use caution when printing or displaying
keywords (Refer to Security
considerations,
below, Chapter 17
).
No specific use for the
segment name is specified in this
document, but applications can use the segment name for such purposes
as constructing a menu of seek points for a slide-show viewer.
It can be included in the optional index that can appear in the
chunk.
It is recommended that the same name not appear in any other
SEEK
chunk or in any
FRAM
or
eXPI
chunk.
Segment names should not begin with the case-insensitive strings
"CLOCK(", "FRAME(", or
"FRAMES(", which are reserved for use in URI
queries and fragments
(see Uniform Resource Identifier
below
).
Applications must not use any information preceding the
SEEK
chunk, except for:
Data appearing in the
MHDR
chunk.
Anything appearing ahead of the
chunk.
They also must not depend on anything that has been drawn on the output
buffer or device. Its contents become undefined when the
SEEK
chunk is encountered. Decoders that make random access to a seek
point from the interior of a segment must insert a background layer before
processing the segment. Encoders must ensure that simple viewers do not
need to do this.
When the
SEEK
chunk is encountered, the decoder can discard
any objects appearing after the
chunk, as though an empty
DISC
chunk were present.
In addition to providing a mechanism for skipping frames or
backspacing over frames, the
SEEK
chunk provides a means
of dealing with a corrupted datastream. The viewer would abandon
processing and simply look for the next
SEEK
chunk before
resuming. Note that looking for a PNG
IHDR
chunk would not
be sufficient because the PNG datastream might be inside a loop or a
Delta-PNG datastream, or it might need data from preceding
MOVE
or
CLIP
chunks.
When a decoder jumps to a seek point from the interior of a segment, it must
restore the information that it saved when it processed the
chunk, and it must reset the object attributes and magnification factors for
object 0 to their default values.
When it encounters a
SEEK
chunk during normal sequential
processing of a MNG datastream, it need not restore anything,
because the encoder will have written chunks that restore all saved information.
Multiple instances of the
SEEK
chunk are permitted. The
SEEK
chunk must not appear prior to the
chunk. The
chunk must also be present if the
SEEK
chunk is present. The
SEEK
chunk must not appear between a
LOOP
chunk and its
ENDL
chunk.
4.5. Ancillary MNG chunks
This section describes ancillary MNG chunks. MNG-compliant decoders
are not required to recognize and process them.
4.5.1.
eXPI
Export image
The
eXPI
chunk takes a snapshot of
a viewable object (either concrete or abstract),
associates the name with that snapshot, and makes
the name available to the "outside world" (like a scripting
language).
The chunk contains an object identifier (snapshot id) and a name:
Snapshot_id: 2 bytes (unsigned integer). Must be zero in
MNG-LC and MNG-VLC datastreams.
Snapshot_name: 1-79 bytes (Latin-1 text).
When the snapshot_id is zero, the snapshot is the first instance
of an embedded image
with object_id=0
following the
eXPI
chunk.
When the
snapshot_id is nonzero, the snapshot is an already-defined object with that
object_id as it already exists when the
eXPI
chunk is encountered.
Note that the
snapshot_name
is associated with the
snapshot, not with the
snapshot_id
nor its subsequent contents;
changing the image identified by
snapshot_id
will not
affect the snapshot.
The
snapshot_name
means nothing inside the scope of the
MNG specification, except that it can be included in
the optional index that can appear in the
chunk.
If two
eXPI
chunks use the same name, it is the outside world's
problem (and the outside world's prerogative to regard it as an error).
It is recommended, however, that the
snapshot_name
not be
the same as that appearing in any other
eXPI
chunk or in any
FRAM
or
SEEK
chunk. A decoder that knows of no
"outside world" can simply ignore the
eXPI
chunk. This
chunk could be used in MNG datastreams that define libraries of related
images, rather than animations, to allow applications to extract
images by their
snapshot_id
Names beginning with the word "thumbnail" are reserved for snapshot
images that are intended to make good icons for the MNG. Thumbnail
images are regular PNG
or Delta-PNG
images, but they would normally have
smaller dimensions and fewer colors than the MNG frames.
They can be defined with the potential visibility field set
to "invisible" if they
are not intended to be shown as a part of the regular display.
The
snapshot_name
string must follow the format of a
tEXt
keyword: It must consist only of printable Latin-1
characters and must not have leading or trailing blanks, but can have
single embedded blanks. There must be at least one and no more than
79 characters in the keyword. Keywords are case-sensitive. There is
no null byte terminator within the
snapshot_name
string,
nor is there a separate null byte terminator. Snapshot names should
not begin with the case-insensitive strings
"CLOCK(", "FRAME(", or "FRAMES("
which are reserved for use in URI queries and
fragments (see Uniform Resource Identifier
below
).
Multiple instances of the
eXPI
chunk are permitted
in a MNG datastream, and they need not have different values of
snapshot_id
4.5.2.
fPRI
Frame priority
The
fPRI
chunk allows authors to assign a priority to a
portion of the MNG datastream. Decoders can decide whether or not to
decode and process that part of the datastream based on its "priority"
compared to some measure of "cost".
The
fPRI
chunk contains two bytes:
fPRI_delta_type: 1 byte (unsigned integer).
0: Priority is given directly.
1: Priority is determined by adding the fPRI
data to the previous value, modulo 256.
Priority or delta_priority:
1 byte (signed integer). Value to be assigned to
subsequent chunks until another fPRI chunk is
reached.
While 256 distinct values of
priority
are possible, it
is recommended that only the values 0 (low priority), 128 (medium
priority), and 255 (high priority) be used. Viewers that can only
display a single image can look for one with
priority=255
and
stop after displaying it. If the datastream contains a large number
of frames and includes periodic "initial" frames that do not contain
Delta-PNG datastreams, each "initial" frame could be preceded by a
fPRI
with
priority=128
and followed by one with
priority=0
, and the best representative initial frame could
be preceded by a
fPRI
chunk with
priority=255
. Then
single-image viewers would just display the representative frame, slow
viewers would display just the "initial" frames, and fast viewers
would display everything.
If a viewer has established a nonzero "cost", it must skip any
portion of the datastream whose priority is less than that "cost".
The "cost" must be established prior to processing the proloque
segment. If the decoder changes its "cost" it must process again
according to the new "cost", unless it knows that there were
no
fPRI
chunks in the prologue segment.
The
SEEK
, and
MEND
chunks always
have
priority=255
; decoders must look for these chunks in
addition to the
fPRI
chunk while skipping a low-priority
portion of the datastream.
It is not permissible for a portion of the datastream to depend on
any portion of the datastream having a lower value, because a decoder
might have skipped the lower value portion. Use of the
fPRI
chunk is illustrated in
Example 5
and
Example 9
Viewers that care about the priority must assume
priority=255
for any portion of the MNG datastream that is
processed prior to the first
fPRI
chunk.
Multiple instances of the
fPRI
chunk are permitted.
4.5.3.
nEED
Resources needed
The
nEED
chunk can be used to specify needed resources, to
provide a quick exit path for viewers that are not capable of displaying
the MNG datastream.
The
nEED
chunk contains a list of keywords that the decoder
must recognize. Keywords are typically private critical chunk names.
Keyword: 1-79 bytes.
Separator: 1 byte (null).
...etc...
The
nEED
chunk should be placed early in the MNG datastream,
preferably very shortly after the
MHDR
chunk.
The keywords are typically 4-character private critical chunk
names, but they could be any string that a decoder is required to
recognize. No critical chunks defined in this specification or in the
PNG specification should be named in a
nEED
chunk, because
MNG-compliant decoders are required to recognize all of them, whether
they appear in a
nEED
chunk or not. The purpose of the
nEED
chunk is only to identify requirements that are above and
beyond the requirements of this document and of the PNG specification.
Each keyword string must follow the format of a
tEXt
keyword: It must consist only of printable Latin-1 characters and
must not have leading or trailing blanks, but can have single embedded
blanks. There must be at least one and no more than 79 characters
in the keyword. Keywords are case-sensitive. There is no null byte
terminator within the keyword. A null separator byte must appear after
each keyword in the
nEED
chunk except for the last one.
Decoders that do not recognize a chunk name or keyword in the
list should abandon the MNG datastream or request user intervention.
The normal security precautions should be taken when displaying the
keywords.
4.5.4.
pHYg
Physical pixel size (global)
The MNG
pHYg
chunk is identical in syntax to the PNG
pHYs
chunk. It applies to complete
full-frame MNG layers and not to the individual images within them.
Conceptually, a MNG viewer that processes the
pHYg
chunk will
first composite each image into a full-frame layer, then apply
the
pHYg
scaling to the layer, and finally composite the scaled
layer against the frame.
MNG datastreams can include both the PNG
pHYs
chunk (either at the
MNG top level or within the PNG and JNG datastreams) and the MNG
pHYg
chunk (only at the MNG top level), to ensure that the images
are properly displayed either when displayed by a MNG viewer or
when extracted into a series of individual PNG or JNG datastreams
and then displayed by a PNG or JNG application. The
pHYs
and
pHYg
chunks would normally contain the same values, but this is
not necessary.
The MNG top-level
pHYg
chunk can be nullified by a
subsequent empty
pHYg
chunk appearing in the MNG top level.
4.6. Ancillary PNG chunks
The namespace for MNG chunk names is separate from that of PNG. Only
those PNG chunks named in this paragraph are also defined at the MNG top
level. They have exactly the same syntax and semantics as when they
appear in a PNG datastream:
iTXt
tEXt
zTXt
tIME
Same format as in PNG. Can appear at most once in the
prologue segment (before the first
SEEK
chunk), and at most
once per segment (between two consecutive
SEEK
chunks). In the
prologue it indicates the last time any part of the MNG was modified.
In a regular segment (between
SEEK
chunks or between the final
SEEK
chunk and the
MEND
chunk), it indicates the
last time that segment was modified.
A MNG editor that writes PNG datastreams should not include
the top-level
iTXt
tEXt
tIME
and
zTXt
chunks in the generated PNG datastreams.
cHRM
gAMA
iCCP
sRGB
bKGD
sBIT
pHYs
These PNG chunks are also defined at the MNG top level. They
provide default values to be used in case they are not provided in
subsequent PNG datastreams. Any of these chunks can be nullified by the
appearance of a subsequent empty chunk with the same chunk name. Such
empty chunks are not legal PNG or JNG chunks and must only appear in the
MNG top level.
In the MNG top level, all of these chunks are written as though
for 16-bit RGBA PNG datastreams. Decoders are responsible for
reformatting the chunk data to suit the actual bit depth and color
type of the datastream that inherits them.
A MNG editor that writes PNG or JNG datastreams is expected to
include the top-level
cHRM
gAMA
iCCP
and
sRGB
chunks in the generated PNG or JNG datastreams, if
the embedded image does not contain its own chunks that define the
color space. When it writes the
sRGB
chunk, it should write
the
gAMA
chunk (and perhaps the
cHRM
chunk), in
accordance with the PNG specification, even though no
gAMA
or
cHRM
chunk is present in the MNG datastream.
It is also expected to write the
pHYs
chunk and the reformatted
top-level
bKGD
chunk in the generated PNG or JNG datastreams, and
the reformatted
sBIT
chunk only in generated PNG datastreams, when
the datastream does not have its own
bKGD
pHYs
or
sBIT
chunks.
The top-level
sRGB
chunk nullifies the preceding
top-level
gAMA
and
cHRM
chunks, if any, and
either the top-level
gAMA
or the top-level
cHRM
chunk
nullifies the preceding top-level
sRGB
chunk, if any.
sPLT
This PNG chunk is also defined at the MNG top
level. It
provides a value that takes precedence over those that might be provided
in subsequent PNG or JNG datastreams and provides a value to be used
when it is not provided in subsequent PNG or JNG datastreams.
It also takes precedence over the
PLTE
chunk in a subsequent PNG datastream when the
PLTE
and
hIST
chunks are being used as a suggested
palette (i.e.,
color_type != 3
). This chunk can appear for
any color type. There can be multiple
sPLT
chunks in a MNG
datastream. If a
palette_name
is repeated, the previous
palette having the same
palette_name
is replaced. It is
not permitted, at the MNG top level, to redefine a palette after the
chunk with the same
palette_name
as one that
appears ahead of the
chunk. It is permitted, however, to
define and redefine other palettes with other
palette_name
fields. A single empty
sPLT
chunk can be used to nullify all
sPLT
chunks that have been previously defined in the MNG top
level, except for those that appeared ahead of the
chunk,
when the
chunk has been read.
When a decoder needs to choose between a suggested palette
defined at the MNG level and a suggested palette defined in the PNG datastream
(either with the
sPLT
chunk, or with the
PLTE/hIST
chunks for grayscale or truecolor images), it should give precedence to
the palette from the MNG level, to avoid spurious layer-to-layer color
changes.
MNG editors that write PNG datastreams should ignore the
sPLT
data from the MNG level and simply copy
any
sPLT
chunks appearing within the embedded PNG
datastreams.
5. The JPEG Network Graphics (JNG) Format
JNG (JPEG Network Graphics) is the lossy sub-format for MNG
objects.
MNG-LC and MNG-VLC
applications can choose to support JNG or not. Those that
do not can check bit 4 (JNG is present/absent) of the
MHDR
simplicity profile to decide whether they can process the datastream.
Note: This specification depends on the PNG Portable Network Graphics
specification
PNG
The PNG specification is available at the PNG home page,
A JNG datastream consists of a header chunk (
JHDR
),
JDAT
chunks that contain a complete JPEG datastream,
optional
IDAT
chunks that contain a PNG-encoded grayscale
image that is to be used as an alpha mask, and an
IEND
chunk.
The alpha mask, if present, must have the same dimensions as the image
itself. The
JDAT
and
IDAT
chunks can be interleaved.
Some of the PNG ancillary chunks are also recognized in JNG datastreams.
While JNG is primarily intended for use as a sub-format within MNG, a
single-image JNG datastream can be written in a standalone file. If so,
the JNG datastream begins with an 8-byte signature containing
139 74 78 71 13 10 26 10 (decimal)
8b 4a 4e 47 0d 0a 1a 0a (hexadecimal)
\213 J N G \r \n \032 \n (ASCII C notation)
which is similar to the PNG signature
with "\213 J N G"
instead of "\211 P N G" in bytes 0-3.
We may at some future time register an Internet Media
Type for JNG files. Until then, the interim media type
image/x-jng
can be used. It is recommended that the
file extension ".jng" (lower case preferred) be used.
JNG is pronounced "Jing."
5.1. Critical JNG chunks
This section specifies the critical chunks that are defined in the
JNG format.
5.1.1.
JHDR
JNG header
The format of the
JHDR
chunk introduces a JNG datastream.
It contains:
Width: 4 bytes (unsigned integer, range 0..65535).
Height: 4 bytes (unsigned integer, range 0..65535).
Color_type: 1 byte
8: Gray (Y).
10: Color (YCbCr).
12: Gray-alpha (Y-alpha).
14: Color-alpha (YCbCr-alpha).
Image_sample_depth:
1 byte
8: 8-bit samples and quantization tables.
12: 12-bit samples and quantization tables.
20: 8-bit image followed by a 12-bit image.
Image_compression_method:
1 byte
8: ISO-10918-1 Huffman-coded baseline JPEG.
Image_interlace_method:
1 byte.
0: Sequential JPEG, single scan.
8: Progressive JPEG.
Alpha_sample_depth:
1 byte.
0, 1, 2, 4, 8, or 16, if the Alpha compression method is 0
(PNG)
8, if the Alpha compression method is 8 (JNG).
Alpha_compression_method:
1 byte.
0: PNG grayscale IDAT format.
8: JNG 8-bit grayscale JDAA format.
Alpha_filter_method:
1 byte.
0: Adaptive PNG (see PNG spec) or not applicable (JPEG).
Alpha_interlace_method:
1 byte.
0: Noninterlaced PNG or sequential single-scan JPEG.
The width, height, image_sample_depth, image_compression_method,
and image_interlace_method fields are redundant because equivalent
information is also embedded in the
JDAT
datastream. They
appear in the
JHDR
chunk for convenience. Their values must be
identical to their equivalents embedded in the
JDAT
chunk. We
use four bytes in the width and height fields for similarity to MNG and
PNG, and to leave room for future expansion, even though two bytes would
have been sufficient.
When the
color_type
is 8 or 10 (no alpha channel), the
last four bytes, which describe the
IDAT
or
JDAA
data,
must be set to zero. The
alpha_sample_depth
must be nonzero
when the alpha channel is present.
5.1.2.
JDAT
JNG image data
A JNG datastream must contain one or more
JDAT
chunks, whose
data, when concatenated, forms a complete JNG JPEG datastream.
JNG decoders are required to read all baseline JNG JPEG and
eight-bit progressive JNG JPEG datastreams. Twelve-bit capability is
not required.
JDAT
chunks are like PNG
IDAT
chunks in that
there may be multiple
JDAT
chunks, the data from which
are concatenated to form a single datastream that can be sent to
the decompressor. No chunks are permitted among the sequence of
JDAT
chunks, except for interleaved
IDAT
chunks.
The ordering requirements of other ancillary chunks are the same
with respect to
JDAT
as they are in PNG with respect to the
IDAT
chunk.
A JNG JPEG is a baseline, extended-sequential, or progressive JPEG as
defined by JPEG Part 1
ISO/IEC-10918-1
JNG uses only JFIF-compatible
JFIF
component interpretations, and imposes a few additional restrictions
that reflect limitations of many existing JPEG implementations. In
particular, only Huffman entropy coding is permitted.
Actually, a JNG may contain two separate JNG JPEG datastreams
(one eight-bit and one twelve-bit), each contained in a series
of
JDAT
chunks, and separated by a
JSEP
chunk
(see the
JSEP
chunk specification
below, Paragraph 5.1.5
).
Decoders that are unable to (or do not wish to) handle twelve-bit datastreams
are allowed to display the eight-bit datastream instead, if one is present.
The core of the JNG JPEG definition is baseline JNG JPEG, which
is JPEG Part 1's definition of baseline JPEG further restricted by
JFIF restrictions and JNG-specific restrictions. JNG JPEG also includes
progressive JPEG, which is also defined in JPEG Part 1 and has JNG-specific
restrictions.
Baseline JNG JPEG restrictions
A baseline JPEG according to JPEG Part 1 is DCT-based
(lossy) sequential
JPEG, using 8-bit sample precision and Huffman entropy coding, with the
following further restrictions:
Quantization table precision must be 8 bits for baseline JPEG.
Huffman code tables can have table numbers 0 and 1 only.
The SOF marker type for baseline JPEG is SOF0.
JDAT datastreams must always follow "interchange JPEG" rules: all
necessary quantization and Huffman tables must be included in the
datastream; no tables can be omitted.
JFIF-compatible restrictions
The image data is always stored left-to-right, top-to-bottom.
The encoded data shall have one of the two color space
interpretations allowed by the JFIF specification:
Grayscale: a single component representing luminance, ranging from 0
for black to 255 for white (or 0 to 4095 when dealing with twelve-bit data).
This component shall have JPEG component identifier 1.
YCbCr: three components representing luminance, chroma blue, and
chroma red, in that order. The components shall be assigned JPEG
component identifiers 1, 2, 3 respectively. YCbCr is defined as a
linear transformation from RGB color space:
Y = Luma_red*R + Luma_green*G + Luma_blue*B
Cb = (B - Y) / (2 - 2*Luma_blue) + Half_scale
Cr = (R - Y) / (2 - 2*Luma_red) + Half_scale
By convention, the luminance coefficients are always those
defined by CCIR Recommendation 601-1:
Luma_red = 0.299
Luma_green = 0.587
Luma_blue = 0.114
The constant Half_scale is 128 when dealing with eight-bit
data, 2048
for twelve-bit data. With these equations, Y, Cb, and Cr all have the same
range as R, G, and B: 0 to 255 for eight-bit data, 0 to 4095 for twelve-bit
data.
The JFIF convention for YCbCr differs from typical digital
television practice in that no headroom/footroom is reserved: the coefficient
values range over the full available 8 or 12 bits.
Intercomponent sample alignment shall be such that the
first (upper
leftmost) samples of each component share a common upper left corner
position. This again differs from common digital TV practice, in which
the first samples share a common center position. The JFIF convention
is simpler to visualize: subsampled chroma samples always cover an
integral number of luminance sample positions, whereas with co-centered
alignment, chroma samples only partially overlap some luminance samples.
Additional JNG restrictions
JNG imposes three additional restrictions not found in the
text of either JPEG Part 1 or the JFIF specification:
The sampling factors for YCbCr images must be one of these sets:
1h1v,1h1v,1h1v (also called 4:4:4 or 1x1 sampling)
2h1v,1h1v,1h1v (also called 4:2:2 or 2x1 sampling)
2h2v,1h1v,1h1v (also called 4:2:0 or 2x2 sampling)
1h2v,1h1v,1h1v (also called 1x2 sampling)
In other words, the chroma components may be downsampled 2:1
or 1:2 horizontally or vertically relative to luminance, or they may be left
full size. These four sampling ratios are the only ones supported by a
wide spectrum of implementations (1x2 is relatively uncommon, and is usually
the result of a lossless rotation of a 2x1 sampling).
For grayscale images, the sampling factors are irrelevant
according to a strict reading of JPEG Part 1. Hence decoder authors should
accept any sampling factors for grayscale. However, we recommend that encoders
always emit sampling factors 1h1v for grayscale, since some decoders
have been observed to malfunction when presented with other sampling
factors.
There must be only one scan in an image: that is, YCbCr images
must be fully interleaved. There is little advantage to be gained by
encoding a baseline image in multiple scans, and many baseline decoders
do not support multiple scans at all.
The DNL (Define Number of Lines) marker is prohibited. The image
height must always be specified accurately in the SOFn marker and in the
JHDR
chunk.
Recommended progressive JPEG subset
For JNG progressive JPEG datastreams, the JPEG process is
progressive Huffman coding (SOF marker type SOF2) rather than baseline (SOF0).
All JNG-compliant decoders must support full progression, including both
spectral-selection and successive-approximation modes, with any sequence
of scan progression parameters allowed by the JPEG Part 1 standard.
Otherwise, all the restrictions listed above apply, except
these:
Multiple-scan support is obviously required for progressive JPEG.
Huffman table numbers up to 3 (the full JPEG limit) may be used,
since the baseline two-table limit is unlikely to be needed by any
decoder that can handle progressive JPEG.
We require full progression support since relatively little
code savings can be achieved by subsetting the JPEG progression features.
In particular, successive approximation offers significant gains in
the visual quality of early scans. Omitting successive-approximation
support from a decoder does not save nearly enough code to justify
restricting JNG progressive encoders to spectral selection only.
No particular progressive scan sequence is specified or
recommended by this specification. Not enough experience has been gained with
progressive JPEG to warrant making such a recommendation. To allow
for future experimentation with scan sequences, decoders are expected
to handle any JPEG-legal sequence. Again, the code savings that might
be had by making restrictive assumptions are too small to justify a
limitation.
When the
JSEP
chunk is present, both images must be
progressive if one of them is progressive.
Recommended 12-bit JPEG subset
JNG JPEGs may optionally use 12-bit sample precision as
defined in JPEG Part 1.
For a sequential image, the SOF marker type must be SOF1
(extended sequential) not SOF0, and the baseline restriction of two Huffman
tables is removed. Also, the encoder may use either 8-bit or 16-bit
quantization tables. All other JNG baseline restrictions still apply.
It is recommended that JNG encoders not use extended-sequential mode
except to encode 12-bit data.
For a progressive image, the only difference between
8-bit and 12-bit
modes is that the sample precision is 12 bits and the encoder may use
either 8-bit or 16-bit quantization tables. All other JNG restrictions
still apply.
5.1.3.
IDAT
JNG PNG-encoded alpha data
This chunk is exactly like the
IDAT
chunk in a PNG grayscale
image, except that it is interpreted as an alpha mask to be applied to
the image data from the
JDAT
chunks,
when
alpha_compression_method=0
. The alpha channel, if
present, can have sample depths 1, 2, 4, 8, or 16.
The filter method can be any filter method that is defined for PNG
datastreams that are embedded in MNG datastreams.
The
IDAT
chunks can be interleaved with the
JDAT
chunks
(see Recommendations for Encoders:
JNG interleaving
below
).
No other chunk
type can appear among the sequence of
IDAT
and
JDAT
chunks. No other chunk type can appear between the sequences of
IDAT
and
JDAT
chunks when they are not interleaved.
The samples in the IDAT must be presented in noninterlaced order, left
to right, top to bottom. As in PNG, zero means fully transparent and
alpha_sample_depth
-1
means fully opaque.
The
IDAT
chunks must precede the
JSEP
chunk, if the
JSEP
chunk is present. Minimal viewers that ignore the twelve-bit
JDAT
chunks must read the
IDAT
chunks and apply the
alpha samples to the eight-bit image that is contained in the
JDAT
chunks that precede the
JSEP
chunk.
Viewers that skip the eight-bit
JDAT
chunks must decode the
IDAT
chunks that precede the
JSEP
chunk and apply the
alpha samples to the twelve-bit image that is contained in the
JDAT
chunks that follow the
JSEP
chunk.
5.1.4.
JDAA
JNG JPEG-encoded alpha data
This chunk is exactly like the
JDAT
chunk in a non-progressive
JNG 8-bit grayscale image, except that it is interpreted as an alpha mask to
be applied to the image data from the
JDAT
chunks,
when
alpha_compression_method=8
. The alpha channel, if
present, can have only sample depth 8. The
JDAA
chunks can be interleaved with the
JDAT
chunks
(see Recommendations for Encoders:
JNG interleaving
below
).
Like
IDAT
chunks, the
JDAA
chunks must precede
the
JSEP
chunk, if the
JSEP
chunk is present, and are
handled similarly.
5.1.5.
JSEP
8-bit/12-bit image separator
JNG permits storage of both an 8-bit and a 12-bit JPEG datastream in a
single JNG file. This feature allows an 8-bit image to be provided for
non-12-bit-capable decoders. The
JSEP
chunk is used to separate
the two datastreams.
The
JSEP
chunk is empty.
JSEP
chunk must appear between the
JDAT
chunks of an eight-bit datastream and those of a twelve-bit datastream, when
image_sample_depth=20
in the
JHDR
chunk. When
image_sample_depth != 20
, the
JSEP
chunk must not
be present. The eight-bit
datastream must appear first. Both images must have the same width,
height, color type, compression method, and interlace method. Viewers can
choose to display one or the other image, but not both.
5.1.6.
IEND
End of JNG datastream
The JNG
IEND
chunk is identical to its counterpart in
PNG. Its data length is zero, and it serves to mark the end of the JNG
datastream.
5.2. Ancillary JNG chunks
Some PNG ancillary chunks can also appear in JNG datastreams, and are
used for the same purposes as described in the PNG specification
PNG
and the Extensions to the PNG Specification document
PNG-EXT
If the
bKGD
chunk is present, it must be written as if it
were written for a PNG datastream with sample_depth=8. It has one
2-byte entry for grayscale JNGs and three 2-byte entries for color JNGs.
The first (most significant) byte of each entry must be 0.
The following chunks have exactly the same meaning and have the same
syntax as given in the PNG specification:
cHRM
gAMA
iCCP
sRGB
pHYs
oFFs
sCAL
If they are present, they must appear prior to the first
JDAT
chunk.
The following chunks also have the same meaning and syntax as in PNG:
iTXt
tEXt
tIME
, and
zTXt
. They
can appear prior to the first or after the last
JDAT
chunk.
The PNG
PLTE
hIST
pCAL
sBIT
sPLT
tRNS
fRAc
, and
gIF*
chunks are
not defined in JNG.
When
cHRM
gAMA
iCCP
, or
sRGB
are present, they provide information about the color space of the
decoded
JDAT
image, and they have no effect on the decoded
alpha samples from the
IDAT
or
JDAA
chunks. Any viewer that
processes the
gAMA
chunk must also recognize and process the
sRGB
chunk. It can treat it as if it were a
gAMA
chunk containing the value .45455 and it can ignore
its "intent" field.
The chunk copying and ordering rules for JNG are the same as those in
PNG, except for the fact that the
JDAT
chunks and
IDAT
or
JDAA
chunks can be interleaved.
6. The Delta-PNG Format
A Delta-PNG datastream describes a single image, by giving the
changes from a previous PNG (Portable Network Graphics) image or
nonviewable PNG-like object, a JNG (JPEG Network Graphics) image, or
another Delta-PNG image.
No provision is made in this specification for storing a Delta-PNG
datastream as a standalone file. A Delta-PNG datastream will normally
be found as a component of a MNG datastream. Applications that need
to store a Delta-PNG datastream separately should use a different
file signature and filename extension, or they can wrap it in a MNG
datastream consisting of the MNG signature, the
MHDR
chunk, a
BASI
chunk with the appropriate dimensions and an
IEND
chunk, the Delta-PNG datastream, and the
MEND
chunk.
The decoder must have available a parent (decoded) object
that has an object buffer from which
the original chunk data is known. The parent object can be the result
of decoding a PNG, another Delta-PNG datastream, or it could have been
generated by a PNG-like datastream introduced by a
BASI
chunk.
The child image is always of the same basic type (at present only
PNG and JNG are defined) as the parent object. The child is always a
viewable image even if the parent is not.
The decoder must not have modified the pixel data in the parent
object by applying output transformations such as
gAMA
or
cHRM
, or by compositing the image against a background.
Instead, the decoder must make available to the Delta-PNG decoder the
unmodified pixel data along with the values for the
gAMA
cHRM
, and any other recognized chunks from the parent object
datastream.
A Delta-PNG datastream consists of a
DHDR
and
IEND
enclosing other optional chunks (if there are no other chunks, the
decoder simply copies the parent image, and displays it if its
do_not_show=0
).
Chunk structure (length, name, CRC) and the chunk-naming system
are identical to those defined in the PNG specification. Definitions
of
compression_method
and
interlace_method
are
also the same as defined in the PNG specification. The definition
of
filter_method
is the same as for PNG datastreams that
are embedded in MNG datastreams
(see the
IHDR
chunk specification,
above, Paragraph 4.2.3
).
6.1. Delta-PNG critical chunks
This section describes critical Delta-PNG chunks. MNG-compliant
decoders must recognize and process them.
6.1.1.
DHDR
Delta-PNG datastream header
The
DHDR
chunk introduces a Delta-PNG datastream.
Subsequent chunks, through the next
IEND
chunk, are interpreted
according to the Delta-PNG format.
The
DHDR
chunk can contain 4, 12, or 20 bytes:
Object_id: 2 bytes (nonzero unsigned integer). Identifies the parent
object from which changes will be made. This is also the
object_id of the child image, which can be used as the
parent image for a subsequent Delta-PNG.
Image_type: 1 byte.
0: Image type is unspecified. An IHDR, JHDR, IPNG, or
IJNG chunk must be present. If JHDR or IJNG is
present, delta_type must not be 1, 3, 4, or 6.
1: Image type is PNG. IHDR and IPNG can be omitted under
certain conditions.
2: Image type is JNG. JHDR and IJNG can be omitted under
certain conditions. Delta_type must not be 1, 3, 4,
or 6.
Delta_type: 1 byte.
0: Entire image replacement.
1: Block pixel addition, by samples, modulo 2^sample_depth.
2: Block alpha addition, by samples, modulo 2^sample_depth.
Regardless of the color type of the parent image, the
IDAT data are written as a grayscale image (color type
0), but the decoded samples are used as deltas to the
alpha samples in the parent image. The parent image
must have (or be promoted to via the PROM chunk) a
color type that has an alpha channel.
3: Block color addition. Similar to delta type 1 except
that only the color channels are updated even when the
parent has an alpha channel.
4: Block pixel replacement.
5: Block alpha replacement.
6: Block color replacement.
7: No change to pixel data.
Block_width: 4 bytes (unsigned integer). This field must be omitted
when delta_type=7.
Block_height: 4 bytes (unsigned integer). This field must be omitted
when delta_type=7.
Block_X_location:
4 bytes (unsigned integer), measured in pixels from the
left edge of the parent object. This field must be
omitted when delta_type=0 or when delta_type=7.
Block_Y_location:
4 bytes (unsigned integer), measured in pixels from the
top edge of the parent object. This field must be
omitted when delta_type=0 or when delta_type=7.
The
object_id
must identify an existing object, and the
object must be a "concrete" object, i.e., it must have the property
concrete_flag=1
The
image_type
, whether given explicitly as 1 or 2
or implied by the presence of an
IHDR
IPNG
JHDR
, or
IJNG
chunk, must be the same as that of the
parent object.
When
delta_type=0
, the width and height of the child image
are given by the
block_width
and
block_height
fields.
For all other values of
delta_type
, the width and height
of the child image are inherited from the parent object.
When
delta_type=1-6
, the
block_width
and
block_height
fields give the size of the block of
pixels to be modified or replaced, and
block_X_location
and
block_Y_location
give its location with respect to the top
left corner of the parent object. The block must fall entirely within
the parent object.
Entire image replacement
When
delta_type=0
in the
DHDR
chunk, the
pixel data in the
IDAT
chunks represent a completely new
image, with dimensions given by the
block_width
and
block_height
fields of the
DHDR
chunk. Data from
chunks other than
IDAT
or
JDAT
can be inherited from
the parent object. If the
IHDR
or
JHDR
chunk is present,
all of its fields except
width
and
height
(which must be ignored by decoders) provide
new values that are inherited by subsequent objects. The "pixel
sample depth" and "alpha sample depth" are also reset
equal to the
IHDR
sample_depth
value (in the case of a JNG object,
the new "alpha sample depth" is taken from the
JHDR
alpha_sample_depth
field). If the
IHDR
or
JHDR
chunk is not present,
the
IDAT
chunks are decoded according to the parent object's
sample depth, and not according to the "pixel
sample depth" or "alpha sample depth" which are used for
decoding the
IDAT
chunks in subsequent Delta-PNG datastreams
when
delta_type
is nonzero.
Block pixel addition
When
delta_type=1
in the
DHDR
chunk, the pixel
data in the
IDAT
chunks represent deltas from the pixel data in
a parent object known to the decoder, including the alpha channel, if the
parent object has an alpha channel.
The
IDAT
chunk data contains a filtered and perhaps
interlaced set of delta pixel samples. The delta samples are presented
in the order specified by
interlace_method
, filtered
according to the
filter_method
and compressed according to
the
compression_method
given in the
IHDR
chunk.
The pixel data includes alpha
samples, if the parent object has an alpha channel.
An encoder calculates the delta sample values from the samples
in the
parent object and those in the child image by subtracting the parent
object samples from the child image samples, modulo
sample_depth
When decoding the
IDAT
chunk, the child image bytes are
obtained by adding the delta bytes to the parent object bytes, modulo
sample_depth
. This is similar in operation to the PNG SUB filter,
except that it works by samples instead of working by bytes.
Only the pixels
in the block defined by the block location and dimensions given in the
DHDR
chunk are changed. The size of the
IDAT
data
must correspond exactly to this rectangle.
When the parent object has
color_type=3
, the
deltas are differences between index values, not between color samples.
The color type must match that of the parent,
except that
when the parent has PNG
color_type=3
, the delta can have
color_type=0
, and vice versa, since the contents of
the
IDAT
chunks of either color type are indistinguishable.
If the
pixel_sample_depth
does not match the
object_sample_depth
, the delta must be scaled to the
object_sample_depth
using the zero-fill or right-shift method
described in the PNG specification, before performing the pixel addition.
When the
IHDR
chunk is present, the compression
method, filter method, and interlace method
need not be the same as those of the parent object. The new
values are used in decoding the IDAT data, and the new values are
inherited by the child object.
Whenever the sample depth differs from that of
the parent object, the resulting object inherits the original value from
the parent. The value from the
IHDR
chunk is
only used for decoding the
IDAT
data in this and subsequent
Delta-PNGs. Implicit in this is the requirement for decoders to remember
in the object buffer
not only the sample depth of the object but (separately) the
"pixel sample depth"
for use in decoding the
IDAT
chunks
of subsequent Delta-PNG datastreams that do not contain their own
IHDR
chunk.
The parent object cannot have alpha samples that were carried in JPEG-encoded
JDAA
chunks.
Block alpha addition
When
delta_type=2
in the
DHDR
chunk, the pixel
data in the
IDAT
chunks represent deltas from the alpha
data in a parent object known to the decoder. The color samples
are not changed, and the updated alpha samples are calculated in
the same manner as the updated pixel samples are calculated when
delta_type=1
The
color_type
is 0 (grayscale), regardless of the
color_type
of the parent object. The parent object must have
an alpha channel or must have been promoted to a type that has an alpha
channel. The compression method, filter method, and interlace method
need not be the same. If they are different, the child object inherits
the new values, and the new values will be used in decoding the data in
any subsequent
IDAT
chunks. Neither the parent object nor the
delta object can have alpha samples that were carried in JPEG-encoded
JDAA
chunks.
The
sample_depth
value from the
IHDR
chunk is
interpreted as a new value of
alpha_sample_depth
and is
only used for decoding the
IDAT
data in this and subsequent
Delta-PNGs. Implicit in this is the requirement for decoders to remember
in the object buffer not only the sample depth of the object
but (separately) the
alpha_sample_depth
for use in decoding the
IDAT
chunks in any subsequent Delta-PNG
datastreams.
If the
alpha_sample_depth
does not match the
object_sample_depth
, the delta must be scaled to the
object_sample_depth
using the zero-fill or right-shift method described in the PNG specification,
before performing the pixel addition.
Block color addition
delta_type=3
is similar to
delta_type=1
except that the alpha channel is not included in the
IDAT
pixels; the alpha channel is inherited from the parent object.
The color type of the parent must be one that has an alpha channel
(4 or 6) and the color type of the delta must be the corresponding color type
(0 or 2) that does not have an alpha channel.
Block pixel replacement
When
delta_type=4
in the
DHDR
chunk, the pixel
data in the
IDAT
chunks represent replacement values for the
pixel samples in the rectangle given by the block location and dimension
fields in the
DHDR
chunk, including the alpha channel, if the
parent object has an alpha channel.
If the
pixel_sample_depth
does not match
the
object_sample_depth
the pixel data must be scaled to the
object_sample_depth
before making the replacements,
using the left bit replication method described in the PNG specification,
or by the right shift method in the unlikely event that the
pixel_sample_depth
is larger than
the
object_sample_depth
The color type must match that of the parent,
except for the cases mentioned for delta type 1, above.
Block alpha replacement
When
delta_type=5
in the
DHDR
chunk, the pixel
data in the
IDAT
chunks represent replacement values of the
alpha samples in the rectangle given by the block location and dimension
fields in the
DHDR
chunk. The sample depth of the
data (i.e. the "alpha sample depth") need not match the sample depth
of the parent object, and
color_type
is
0 (grayscale), regardless of the
color_type
of the parent
object.
If the sample depths differ, the samples must be scaled
to the
object_sample_depth
, using
the left bit replication method or right shift method described
in the PNG specification, depending on whether
the
alpha_sample_depth
is larger or smaller than
the
object_sample_depth
The parent object must have an alpha channel or must have been
promoted to a type that has an alpha channel. The compression method,
filter method, and interlace method need not be the same. If they
differ, the child object inherits the new values.
It is permitted to use JPEG-encoded
JDAA
chunks to convey
the new alpha data. If this is done, then the alpha channel of the object
can no longer be used as the parent for block-pixel-addition or
block-alpha-addition.
Block color replacement
delta_type=6
is similar to
delta_type=4
except that the alpha channel is not included in the
IDAT
pixels; the alpha channel is inherited from the parent object.
The color type of the parent must be one that has an alpha channel
(4 or 6) and the color type of the delta must be the corresponding color type
(0 or 2) that does not have an alpha channel.
No change to pixel data
When
delta_type=7
in the
DHDR
chunk, there is
no change to the pixel data, and it is an error for
IDAT
JDAT
, or
JDAA
to appear. If the
IHDR
or
JHDR
chunk appears, the width, height, and color_type fields are
ignored, the PNG sample depth
(or JNG alpha_sample_depth) is used to update the
pixel_sample_depth
and
alpha_sample_depth
, and the data in the remaining fields
are inherited by the child object.
Pixel sample depth, alpha sample depth
As mentioned above, the sample depth of the deltas
is not necessarily the same as that of the object, when
delta_type
is 0. The decoder needs to remember the
pixel_sample_depth
and
alpha_sample_depth
to use with each
object. They are initialized to the
sample_depth
value
from the
IHDR
chunk that appears when the object is first
created but can be changed by the appearance of the
IHDR
chunk in
a Delta-PNG datastream that has a nonzero
delta_type
If the object is a JNG image, they are initialized from the value
of
alpha_sample_depth
from
the original
JHDR
chunk, and can be changed by the appearance
of the
JHDR
chunk in a Delta-PNG datastream that has
delta_type != 0
6.1.2.
IDAT, JDAT
, and
JDAA
New pixel data
In a Delta-PNG datastream, new pixel data is conveyed by
IDAT
JDAT
, or
JDAA
chunks, depending on the image type and
delta type in the
DHDR
chunk. Any remaining part of the Delta-PNG
datastream following these chunks must be interpreted as PNG or JNG chunks
and not as Delta-PNG chunks. If the image type is 0 (i.e., unspecified),
the first
IDAT
or
JDAA
chunk must be preceded by
an
IHDR
JHDR
IPNG
IJNG
PLTE
, or
PPLT
chunk that will serve to identify the image type.
6.1.3.
PROM
Promotion of parent object
This chunk is used to "promote" a parent object to a higher
bit depth or to add an alpha channel, before making changes to it.
New color type: 1 byte.
New sample depth: 1 byte.
Fill method: 1 byte.
0: Left-bit-replication
1: Zero fill
When a decoder encounters the
PROM
chunk, it must promote
the pixel data. The cases are:
G -> GA (
color_type 0 -> 4
Do not change the gray values. Set all the alpha values to fully
opaque, except for pixels marked transparent by cheap transparency--set
their alpha values to fully transparent. Discard the cheap transparency
information (the PNG
tRNS
chunk data).
RGB -> RBGA (
color_type 2 -> 6
Do not change the RGB values. Convert the
tRNS
chunk data to
alpha values as in the G -> GA promotion.
G -> RGB (
color_type 0 -> 2
Set R, G, and B equal to the gray value. Apply the same operation to
the cheap transparency data (if any). Expand any
bKGD
or
sBIT
data.
GA -> RGBA (
color_type 4 -> 6
Set R, G, and B equal to the gray value. Do not change the alpha
values. Expand any
bKGD
or
sBIT
data.
G -> RGBA (
color_type 0 -> 6
Set R, G, and B equal to the gray value. Handle transparency as in
the G -> GA promotion. Expand any
bKGD
or
sBIT
data.
indexed -> RGB (
color_type 3 -> 2
Set R, G, and B according to the palette entry corresponding to the
index. Discard the cheap transparency information (if any).
Expand any
bKGD
or
sBIT
data.
indexed -> RGBA (
color_type 3 -> 6
Set R, G, and B as in indexed -> RGB. Set the alpha value
according to the cheap transparency information (if any). Discard the
cheap transparency information.
Expand any
bKGD
or
sBIT
data.
JNG-G -> JNG-C (
JNG color_type 8 -> 10
Expand the gray values to color. Expand any
bKGD
data.
JNG-G -> JNG-GA (
JNG color_type 8 -> 12
Do not change the gray values. Set all the alpha values to fully
opaque. The given sample depth is the new sample depth for the alpha
channel.
JNG-G -> JNG-CA (
JNG color_type 8 -> 14
Expand the gray values to color. Set all the alpha values to fully
opaque. The given sample depth is the new sample depth for the alpha
channel. Expand any
bKGD
data.
JNG-C -> JNG-CA (
JNG color_type 10 -> 14
Do not change the color values. Set all the alpha values to fully
opaque. The given sample depth is the new sample depth for the alpha
channel.
JNG-GA -> JNG-CA (
JNG color_type 12 -> 14
Expand the gray values to color. Do not change the alpha values.
Expand any
bKGD
data.
No change in
color_type
Only the sample depth is changed. The new sample depth must be
larger than the old one.
If the sample depth has been changed, the sample values must be
widened. The decoder must use left-bit-replication or zero-fill
according to the specified
fill_method
to fill the additional
bits of each sample. If cheap transparency information is present in a
grayscale or truecolor object, its sample values must also be widened in
the same manner. If the image type is JNG, then the new sample depth
refers to the
alpha_sample_depth
and only the alpha channel
is affected, if one is present. If the
color_type
has been
promoted from indexed-color, the original bit depth is always considered to be
8. See the PNG specification
PNG
for
further information on these filling methods. Any alpha channel added
in this manner is eligible to be updated by block-alpha-addition in this
or a subsequent Delta-PNG.
If the basis object contains data from the PNG
bKGD
chunk,
this data must be promoted as well. If a grayscale object is being
promoted to a truecolor object, the background RGB samples are set equal
to the grayscale background sample. If the bit depth has been changed,
the background samples are widened in accordance with the specified
fill_method
. If the basis object is a JNG, the
bKGD
chunk is not affected.
If the basis object contains data from the PNG
sBIT
chunk,
this data must also be promoted. If a grayscale object is being
promoted to a truecolor object, the new RGB bytes are set equal to the
grayscale byte. When an alpha channel is added, the alpha byte is set
equal to the sample depth of the basis image. If the sample depth has
been changed, the
sBIT
bytes do not change.
The
PROM
chunk is not permitted to "demote" a parent
object to an object with a lesser bit depth or from one with an alpha channel
to one without an alpha channel.
The
PROM
chunk must appear ahead of the
IHDR
chunk,
if
IHDR
is present, and ahead of any chunks that would have
followed
IHDR
, if
IHDR
is omitted.
6.1.4.
IHDR
PNG image header
Inside a Delta-PNG datastream, the
IHDR
chunk introduces
an incomplete PNG (Portable Network Graphics) datastream. The parent
object must be a PNG or PNG-based Delta-PNG. The datastream can be
introduced by a complete PNG
IHDR
chunk or by an
IPNG
chunk, which is empty.
If the
IHDR
chunk is present, its
width
and
height
fields are ignored. The values for these parameters are inherited from
the parent object or from the
DHDR
chunk.
The
sample_depth
color_type
compression_method
interlace_method
, and
filter_method
fields, if different from those of the parent
object, are used in decoding any subsequent
IDAT
chunks, and
the new values will be inherited by any subsequent image that uses this
object as its parent. These do not change the
sample_depth
and
color_type
of the object itself; those can only be
changed by using the
PROM
chunk or by using
delta_type=0
See the PNG specification
and the Extensions to the PNG Specification document
PNG-EXT
for the format of the PNG chunks.
The
filter_method
can be any
filter_method
that is
allowed in PNG datastreams that are embedded in a MNG datastream.
The
PNG datastream must contain at least
IHDR
and
IEND
(whether actually present in the datastream or omitted and included
by implication, as described below), but can inherit other chunk data
from the parent object. Except for
IDAT
and
PPLT
any chunks appearing between
IHDR
and
IEND
are always
treated as replacements or additions and not as deltas.
6.1.5.
IPNG
Incomplete PNG
The
IPNG
chunk is empty.
The
IPNG
chunk can be used instead of the
IHDR
chunk if the
IHDR
chunk is not needed for resetting the
value of
compression_method
filter_method
, or
interlace_method
. The purpose of this chunk is to identify
the beginning of the PNG datastream, so decoders can start interpreting
PNG chunks instead of Delta-PNG chunks. The decoder must treat this
datastream as though the
IHDR
chunk were present in the
location occupied by the
IPNG
chunk.
The
IHDR
chunk can also be omitted when
image_type=1
and the PNG datastream begins with a
PLTE
chunk, a
PPLT
chunk, or an
IDAT
chunk.
In this case, no
IPNG
chunk is required, either. The decoder
must treat this datastream as though the
IHDR
chunk were
present, immediately preceding the first PNG chunk. If the first PNG
chunk is neither a
PLTE
chunk, a
PPLT
chunk, nor an
IDAT
chunk, then either the
IPNG
or
IHDR
chunk
must be present to introduce the PNG datastream.
6.1.6.
PLTE
and
tRNS
If the
PLTE
chunk is present, it need not have the same
length as that inherited from the parent object, but it must contain
the complete palette needed in the child image. If it is shorter than
the palette of the parent object, decoders can discard the remaining
entries and the child image must not refer to them. Decoders can also
truncate any
tRNS
data inherited from an indexed-color parent
object. If the new palette is longer than the parent palette, and
a new
tRNS
chunk is not present in an indexed-color image,
the
tRNS
data must be extended with opaque entries. The new
palette must not be longer than the object's
sample_depth
would allow, and must not have more than 256 entries.
When processing the
tRNS
chunk, if
color_type=3
and
PLTE
is not supplied, then the number of allowable entries
is determined from the number of
PLTE
entries in the parent
object. A
tRNS
chunk appearing in a Delta-PNG datastream is
always treated as a complete replacement for the
tRNS
chunk
data in the parent object. All entries beyond those actually supplied
are overwritten with the "opaque" value (255).
6.1.7.
PPLT
Partial palette
If it is desired only to overwrite or add palette entries,
the
PPLT
chunk can be used. This might be useful for
palette-animation applications. This chunk can also be used to
overwrite or add entries to the transparency (alpha) data from the
parent's
tRNS
chunk.
The
PPLT
chunk contains a
delta_type
byte and one
or more groups of palette entries:
PPLT_delta_type: 1 byte.
0: Values are replacement RGB samples.
1: Values are delta RGB samples.
2: Values are replacement alpha samples.
3: Values are delta alpha samples.
4: Values are replacement RGBA samples.
5: Values are delta RGBA samples.
First_index,
first group: 1 byte.
Last_index,
first group: 1 byte.
First set of
samples: 1, 3, or 4 bytes.
...etc...
Last set of
samples: 1, 3, or 4 bytes.
First index,
second group: 1 byte.
...etc...
The
last_index
must be equal to or greater than
first_index
. The groups are not required to appear in
ascending order. If any index of any group is beyond the end of the
parent object's palette, the palette and
tRNS
data must be
extended just as if a longer complete
PLTE
chunk had appeared.
If there are gaps in the resulting extended palette, the colors must be
filled with {0,0,0} and the alphas filled with 255. If alpha samples
are supplied (
PPLT_delta_type > 1
) and no
tRNS
data is present in the parent object, a
tRNS
chunk must be
created in the child object as though a complete
tRNS
chunk
had appeared. The new palette must not be longer than the object's
sample_depth
would allow.
When
PPLT_delta_type=0
, the values are replacements for
the existing samples in the palette.
When
PPLT_delta_type=1
, the values are added to the
existing samples (modulo 256) to obtain the new samples.
If the new entry is beyond the range of the original palette,
the values are simply appended, regardless of the contents of
PPLT_delta_type
6.1.8.
JHDR
JNG image header
Inside a Delta-PNG datastream, the
JHDR
chunk introduces an
incomplete JNG (JPEG Network Graphics) datastream. The parent object
must be a JNG or JNG-based Delta-PNG. The datastream is introduced by a
complete
JHDR
chunk.
If the
JHDR
chunk is present, its
width
height
image_sample_depth
image_color_type
image_filter_method
, and
image_interlace_method
fields are ignored. The values for these
parameters are inherited from the parent object.
The
alpha_compression_method
alpha_interlace_method
, and
alpha_filter_method
fields,
if different from those of the parent object, are used in decoding any
subsequent
IDAT
chunks, and the new values will be inherited by
any subsequent image that uses this object as its parent. If the
alpha_sample_depth
differs, it will be used in decoding the
IDAT
chunk data of the Delta-PNG and subsequent Delta-PNG
datastreams; but the child object itself will retain the original sample depth,
and must also retain the "alpha sample depth" for use in decoding
subsequent Delta-PNG datastreams. The decoded alpha samples must be
scaled to the object's sample depth before the replacements or delta
calculations are done.
See the JNG specification above for the format of the JNG
chunks. The PNG datastream must contain at least
JHDR
and
IEND
, but can inherit other chunk data from the parent
object. Except for IDAT, any chunks appearing between
JHDR
and
IEND
are always treated as replacements or additions and not as
deltas.
6.1.9.
IJNG
Incomplete JNG
The
IJNG
chunk is empty.
The
IJNG
chunk can be used instead of the
JHDR
chunk if the
JHDR
chunk is not needed for resetting the value
of any of the
JHDR
fields. The purpose of this chunk is to
identify the beginning of the JNG datastream, so decoders can start
interpreting JNG chunks instead of Delta-PNG chunks. The decoder must
treat this datastream as though the
JHDR
chunk were present in
the location occupied by the
IJNG
chunk.
The
JHDR
chunk can also be omitted when
image_type=2
and the JNG datastream begins with a
JDAT
or
JDAA
chunk.
In this case, no
IJNG
chunk is required, either. The decoder must treat this datastream as
though the
JHDR
chunk were present, immediately preceding
the first
JDAT
chunk. If the first JNG chunk is not a
JDAT
or
JDAA
chunk, then either the
IJNG
or
JHDR
chunk must be present to introduce the JNG datastream.
6.1.10.
DROP
Drop chunks
All chunks in the parent object with the specified name are inhibited
from being copied into the child image.
The
DROP
chunk contains a one or more 4-byte chunk names:
Chunk_name: 4 bytes (ASCII text).
etc.
Multiple
DROP
chunks are permitted in a Delta-PNG datastream.
If multiple names appear in the
DROP
chunk, it is shorthand
for multiple
DROP
chunks.
6.1.11.
DBYK
Drop chunks by keyword
The
DBYK
chunk contains one or more sequences, each containing
a chunk name, polarity byte, and a keyword:
Chunk_name: 4 bytes (ASCII text).
Polarity: 1 byte (unsigned integer).
0: Only.
1: All-but.
Keywords (null-separated Latin-1 text strings).
etc.
The chunk name must be the name of a chunk whose data begins with
a null-terminated text string. Some parent object chunks with the
specified chunk name are inhibited from being copied into the child
image. If polarity is
keyword appears in the keywords list is inhibited. If polarity is
appear in the keywords list is inhibited.
The format of the keyword is the same as that specified for the
parent chunk. Comparisons of keywords in the parent chunk and the
DBYK
chunk are case sensitive.
Use caution when printing or displaying keywords (Refer to Security
considerations,
below, Chapter 17
).
6.1.12.
ORDR
Ordering restrictions
The
ORDR
chunk informs the applier of the Delta-PNG of the
ordering restrictions for ancillary chunks. It contains one or more
5-byte sequences:
Chunk_name: 4 bytes (ASCII text).
Order_type: 1 byte.
0: Anywhere.
1: After IDAT and/or JDAT or JDAA.
2: Before IDAT and/or JDAT or JDAA.
3: Before IDAT but not before PLTE.
4: Before IDAT but not after PLTE.
etc.
Critical chunk names must not appear in the
ORDR
chunk. The
applier needs to know everything about them anyway.
If a chunk name appears in the
ORDR
chunk, it is a promise
that any chunk of that name appearing in the parent object which is not
inhibited by
DROP/DBYK
will not be broken by this Delta-PNG,
and therefore the applier must copy it into the child image at a
location compatible with its ordering restrictions.
If any ancillary chunk appears in the parent object, and it is not
inhibited, and its name does not appear in the
ORDR
chunk, then
the applier should copy it into the child only if it knows the chunk
well enough to be sure that it is consistent with the changes made by
the Delta-PNG, and knows where it can be placed in the child. Those
conditions are always true of safe-to-copy chunks.
If any critical chunk defined in neither this specification nor the
PNG specification appears in the parent object or in the Delta-PNG,
it is a fatal error unless the applier knows how to handle it. The
specification of the critical chunk can include provisions for this
scenario.
6.2. Ancillary Delta-PNG chunks
This section describes ancillary Delta-PNG chunks. MNG-compliant
decoders should recognize and process them, but are not required to.
6.2.1.
gAMA, cHRM, iCCP, sRGB
Color space chunks
gAMA
cHRM
iCCP
sRGB
or
similar chunk existing in the parent object would not affect the pixel
data in a concrete object inherited by this Delta-PNG datastream
because they are not used in decoding the pixel data. Applications
are responsible for ensuring that the pixel values that are inherited
from the parent object are the raw pixel data that existed prior to any
transformations that were applied while displaying the parent image.
These color transformations are applied to the resulting pixel data for
display purposes.
6.2.2.
oFFs
and
pHYs
MNG viewers must ignore
oFFs
and
pHYs
chunks that
appear inside a PNG or JNG datastream or are inherited from the MNG top
level. MNG editors are expected to treat them as if they were unknown
copy-safe chunks.
6.2.3. Other ancillary PNG chunks
Any other ancillary PNG chunks that the decoder recognizes
when processing
a PNG datastream should also be recognized and handled when processing
a delta-PNG datastream. Any chunks that it does not recognize should
be processed as instructed by the
ORDR
DROP
and
DBYK
chunks. MNG
viewers are free to ignore any ancillary chunks, while MNG editors should
handle them in accordance with the copying rules.
6.2.4.
IEND
End of Delta-PNG datastream
End of Delta-PNG datastream. An
IEND
chunk must be present
for each
DHDR
chunk in a MNG datastream. A single
IEND
terminates both the Delta-PNG datastream and any embedded PNG or JNG
datastream within it.
The
IEND
chunk is empty.
6.3. Chunk ordering requirements
The PNG specification places ordering requirements on many chunks
with respect to the
PLTE
and
IDAT
chunks. If neither
of these two chunks is present, and the
ORDR
chunk is not
present, known chunks (always including all standard chunks described in
the PNG specification) are considered to have appeared in their proper
order with respect to the critical chunks. Unknown chunks are ordered
as described
above (Paragraph 6.1.12)
7. Extension and Registration
New public chunk types, and additional options in existing
public chunks, can be proposed for inclusion in this specification
by contacting the PNG/MNG specification maintainers at
png-info@uunet.uu.net
png-group@w3.org
or
at
mng-list@ccrc.wustl.edu
New public chunks and options will be registered only if they are of use
to others and do not violate the design philosophy of PNG and MNG. Chunk
registration is not automatic, although it is the intent of the authors
that it be straightforward when a new chunk of potentially wide
application is needed. Note that the creation of new critical chunk
types is discouraged unless absolutely necessary.
Applications can also use private chunk types to carry data that
is not of interest to other applications.
Decoders must be prepared to encounter unrecognized public or
private chunk type codes. If the unrecognized chunk is critical, then
decoders should abandon the segment, and if it is ancillary they should simply
ignore the chunk. Editors must handle them as described in the following
section, Chunk Copying Rules.
8. Chunk Copying Rules
The chunk copying rules for MNG are the same as those in PNG, except
that a MNG editor is not permitted to move unknown chunks across any of
the following chunks, or across any critical chunk in a future version
of this specification that creates or displays an image:
SEEK
IHDR
JHDR
IEND
DHDR
BASI
CLON
PAST
SHOW
MAGN
The copy-safe status of an unknown chunk is determined from the chunk
name, just as in PNG. If bit 5 of the first byte of the name is 0
(Normally corresponding to an uppercase ASCII letter), the unknown chunk
is critical and cannot be processed or copied. If it is 1 (usually
corresponding to a lowercase ASCII letter), the unknown chunk is
ancillary and its copy-safe status is determined by bit 5 of the fourth
byte of the name, 0 meaning copy-unsafe and 1 meaning copy-safe.
If an editor makes changes to the MNG datastream that render unknown
chunks unsafe-to-copy, this does not affect the copy-safe status of any
chunks beyond the next
SEEK
chunk or prior to the previous
one. However, if it makes such changes prior the
chunk,
this affects the copy-safe status of all top-level unknown chunks in the
entire MNG datastream.
Changes to the MHDR chunk do not affect the copy-safe status of any
other chunk.
The
SEEK
, and
TERM
chunks are not
considered to be a part of any segment. Changes to the data in the
or
SEEK
chunks do not affect the copy-safe
status of any other chunks. Adding or removing a
SEEK
chunk
affects the copy-safe status of unknown chunks in the newly-merged
or newly-separated segments. Adding, removing, or changing the
TERM
chunk has no effect on the copy-safe status of any chunk.
As in PNG, unsafe-to-copy ancillary chunks in the top-level MNG
datastream can have ordering rules only with respect to critical chunks.
Safe-to-copy ancillary chunks in the top-level MNG datastream can have
ordering rules only with respect to the
SEEK
SHOW
, and
PAST
chunks,
DHDR-IEND
BASI-IEND
IHDR-IEND
JHDR-IEND
sequences, or with respect to any other
critical "header-end" sequence
that might be defined in the future that could contain
IDAT
or
similar chunks.
The copying rules for unknown chunks inside
IHDR-IEND
BASI-IEND
DHDR-IEND
and
JHDR-IEND
sequences
are governed by the PNG and JNG specifications, and any changes inside
such sequences have no effect on the copy-safe status of any top-level
MNG chunks.
The copy-safe status of chunks inside a
DHDR-IEND
sequence
depends on the copy-safe status of the chunks in its parent object.
9. Minimum Requirements for MNG-Compliant Viewers
This section specifies the minimum level of support that is expected of
MNG, MNG-LC, or MNG-VLC-compliant
decoders, and provides recomendations for viewers that
will support slightly more than the minimum requirements. All critical
chunks must be recognized, but some of them can be ignored after they
have been read and recognized. Ancillary chunks can be ignored, and do
not even have to be recognized.
Anything less than this level of support requires subsetting.
Applications that provide less than minimal MNG
support should check the MHDR "simplicity profile" for the
presence of features that they are unable to support or do not wish to
support. A specific subset, in which
"complex MNG features"
and JNG are absent,
is called
"MNG-LC"
In MNG-LC datastreams, bit 0
of the simplicity profile must be 1 and bits 2 and 4
must be 0.
Another subset is called
"MNG-VLC"
In MNG-VLC datastreams, "simple MNG features"
are also absent, and bit 1 must therefore also be 0.
Subsets are useable when the set of MNG datastreams to be processed
is known to be (or is very likely to be) limited to the feature set in
MNG-LC or MNG-VLC.
Limiting the feature set in a widely-deployed WWW browser to anything
less than MNG with 8-bit JNG support would be highly inappropriate.
Some subsets of MNG
support are listed in the following table, more
or less in increasing order of complexity.
MHDR Profile bits Profile Level of support
31-10 9 8 7 6 5 4 3 2 1 0 value
0 0 0 0 1 0 0 0 0 0 1 65 MNG-VLC without transparency
0 0 1 1 1 0 0 1 0 0 1 457 MNG-VLC
0 0 1 1 1 0 1 1 0 0 1 473 MNG-VLC with JNG
0 0 1 1 1 0 0 1 0 1 1 459 MNG-LC
0 0 1 1 1 0 1 1 0 1 1 475 MNG-LC with JNG
0 0 1 1 1 0 1 1 1 1 1 479 MNG without stored object buffers
0 1 1 1 1 0 0 1 1 1 1 975 MNG without JNG or Delta-PNG
0 1 1 1 1 0 1 1 1 1 1 991 MNG without Delta-PNG
0 1 1 1 1 1 0 1 1 1 1 1007 MNG without JNG
0 1 1 1 1 1 1 1 1 1 1 1023 or 0 MNG
0 1 1 1 1 1 1 1 1 1 1 1023 or 0 MNG with 12-bit JNG support
| | | | | | | | | |
| | | | | | | | | +- Validity
| | | | | | | | +--- Simple MNG features
| | | | | | | +----- Complex MNG features
| | | | | | +------- Transparency
| | | | | +--------- JNG
| | | | +----------- Delta-PNG
| | | +------------- Validity of bits 7,8, and 9
| | +--------------- Semitransparency
| +----------------- Background transparency
+------------------- Stored objects
One reasonable path for an application developer to follow might
be to develop and test the application at each of the following
levels of support in turn:
MNG-VLC,
MNG-LC,
MNG-LC with JNG,
MNG.
An equally reasonable development path might be
MNG-VLC with JNG,
MNG-LC with JNG,
MNG with JNG, but without stored object buffers,
MNG.
On the other hand, a developer
working on an application for storing multi-page fax documents might
have no need for more than "MNG-VLC without transparency".
We are allowing conformant decoders to skip twelve-bit JNGs
because those are likely to be rarely encountered and used only for
special purposes. There is no profile flag to indicate the presence
or absence of 12-bit JNGs.
9.1. Required MNG chunk support
MHDR
The
ticks_per_second
must be supported by animation viewers.
The simplicity profile, frame count, layer count, and nominal play time
can be ignored. Decoders that provide less than minimal support can use
the simplicity profile to identify datastreams that they are incapable
of processing.
MEND
The
MEND
chunk must be recognized but does not require any
processing other than completing the last frame.
Global PLTE and tRNS
Must be fully supported. Bit 1 of the simplicity profile can be
used to promise that these chunks are not present.
LOOP, ENDL
The
iteration_count
must be supported. The
nest_level
should be used as a sanity check but is not
required. When
iteration_min=1
either explicitly or
when it is omitted and the
termination_condition
is not 0 or 4, the
LOOP
chunk
and its
ENDL
chunk can be ignored (bit 2 of the simplicity profile
can be used to promise that this is true for all loops).
DEFI, CLON
Must be fully supported.
All objects can be treated as "concrete" if
the decoder does not wish to take advantage of the distinction between
"abstract" and "concrete".
Bit 2 of the simplicity profile can be used to promise that the
CLON
chunk is not present and that if the
DEFI
chunk is present it only
defines object 0, which does not have an object buffer that needs to be stored.
Bit 1 of the simplicity
profile can be used to promise that the
DEFI
chunk
is not present.
BASI, BACK, MAGN, DISC, PAST
Must be fully supported. Bit 2 of the simplicity profile can be used to
promise that the
BASI
DISC
, and
PAST
chunks are
not present, and that if the
BACK
chunk is present it does not
define a background image. Bit 1 can be used to promise that
the
MAGN
chunk is not present.
FRAM
The
framing_mode
and clipping parameters must be
supported. The
interframe_delay
must be supported
except by single-frame viewers. The
sync_id
and
timeout
data can be ignored. Bit 1 of the simplicity profile
can be used to promise that the
FRAM
chunk is not present.
MOVE, CLIP, SHOW
Must be fully supported. Bit 2 of the simplicity profile can be
used to promise that none of these chunks are present, and bit 9 of
the simplicity profile can be used to promise that the
SHOW
chunk is not present.
SAVE and SEEK
Partial support is required: All existing objects must be marked
"frozen" when the
chunk is processed, so that
unneeded objects can be discarded when the
SEEK
chunk or an empty
DISC
chunk is processed. The
SEEK
chunk must be
processed as if it were an empty
DISC
chunk, as a minimum.
Chunk information need only be "saved" and "restored"
when the viewer
is able to skip or jump to random
SEEK
chunk locations from the
interior of a segment, such as when recovering from a corrupted datastream
or from a segment containing an unknown critical chunk, or when escaping
from a deterministic loop in response to a user request. The
optional index can be ignored. Slide-show controllers may wish to
support
and
SEEK
fully. Bit 2 of the simplicity
profile can be used to promise that the
and
SEEK
chunks can be ignored entirely (because there will be nothing to discard).
TERM
Must be recognized but can be ignored.
9.2. Required PNG chunk support
IHDR, PLTE, IDAT, IEND
All PNG critical chunks must be fully supported. All
values of
color_type
bit_depth
compression_method
filter_method
and
interlace_method
must be supported. Interlacing, as in PNG,
need not necessarily be displayed on-the-fly; the image can be displayed
after it is fully decoded. The alpha-channel must be supported, at
least to the degree that fully opaque pixels are opaque and fully
transparent ones are transparent. It is recommended that alpha be fully
supported. Alpha is not present, or can be ignored because it has no
effect on the appearance of any frame, if bit 3 of the simplicity profile is 0.
Bit 1 of the simplicity profile can be used to promise that only filter methods
defined in the PNG specification are present.
tRNS
The PNG
tRNS
chunk, although it is an ancillary chunk, must be
supported in MNG-compliant viewers, at least to the degree that fully
opaque pixels are opaque and fully transparent ones are transparent. It
is recommended that alpha data from the
tRNS
chunk be fully
supported in the same manner as alpha data from an RGBA image or a JNG
with an alpha channel contained in
IDAT
chunks.
The
tRNS
chunk is not present (or can be ignored because it has no
effect on the appearance of any frame) if bit 3 of the simplicity profile is 0.
Other PNG ancillary chunks
Ancillary chunks other than PNG
tRNS
can be ignored, and do
not even have to be recognized.
Color management
It is highly recommended that decoders support at least the
gAMA
chunk to allow platform-independent color rendering.
If they support the
gAMA
chunk, they must also support the
sRGB
chunk, at least to the extent of interpreting it as
if it were a
gAMA
chunk with gamma value 0.45455.
9.3. Required JNG chunk support
Bit 4 of the simplicity profile can be used to promise that
JNG chunks are not present. Viewers that choose not to support
JNG can check this bit before deciding to proceed.
MNG-compliant decoders must support JNG, but
MNG-LC and MNG-VLC
decoders are not required to support JNG.
JHDR, JDAT, IDAT, JDAA, JSEP, IEND
All JNG critical chunks must be fully supported. All
values of
color_type
bit_depth
compression_method
filter_method
and
interlace_method
must be supported. Interlacing, as in PNG,
need not necessarily be displayed on-the-fly; the image can be displayed
after it is fully decoded. The alpha-channel must be supported, at
least to the degree that fully opaque pixels are opaque and fully
transparent ones are transparent. It is recommended that alpha be fully
supported.
JNG ancillary chunks
All JNG ancillary chunks can be ignored, and do not even have to be
recognized.
JNG image sample depth
Only
image_sample_depth=8
must be supported. The
JSEP
chunk must be recognized and must be used by minimal decoders to select
the eight-bit version of the image, when both eight-bit and twelve-bit versions
are present, as indicated by
image_sample_depth=20
in the
JHDR
chunk. When
image_sample_depth=12
, minimal
decoders are not obligated to display anything. Such decoders can
choose to display nothing or an empty rectangle of the width and height
specified in the
JHDR
chunk.
This can be done by processing the JNG as though a
viewable transparent
BASI
object had appeared:
BASI width height 1 4 0 0 0 0 00 00 00 00 1
IEND
9.4. Required Delta-PNG chunk support
MNG-compliant decoders are required to support Delta-PNG, but
MNG-LC and MNG-VLC decoders are not. Bit 2 or 5 of the simplicity
profile can be used to promise that Delta-PNG datastreams are not
present.
DHDR, PROM, IHDR, IDAT, IPNG, PLTE, tRNS, IEND, PPLT
Must be fully supported if Delta-PNG is supported.
JHDR, JDAT, JDAA, JSEP, IJNG
Must be fully supported if JNG is also supported outside of Delta-PNG
datastreams. Bit 4 of the simplicity profile can be used to promise
that no JNG chunks are present.
DROP, DBYK, ORDR
Can be recognized and ignored. These are only of concern to MNG
editors and to MNG viewers that handle private chunks or chunks that can
be selected by keyword, such as
pCAL
and
iCCP
. If you
decide to support such chunks, then you will also have to support these
three chunks.
Ancillary chunks
Ancillary chunks appearing in Delta-PNG datastreams must be treated
in the same manner as if they appeared in a PNG or JNG datastream. See
the recommendations, above. Note that the PNG
tRNS
chunk must
be supported, despite its being an ancillary chunk in PNG.
10. Recommendations for Encoders
The following recommendations do not form a part of the
specification.
10.1. Use a common color space
It is a good idea to use a single color space for all of the layers
in an animation, where speed and fluidity are more important than
exact color rendition. This is best accomplished by defining a
single color space at the top level of MNG, using
either an
sRGB
chunk or the
gAMA
and
cHRM
chunks and perhaps the
iCCP
chunk, and removing any color space chunks from the individual images
after converting them to the common color space.
When the encoder converts all images to a single color space before
putting them in the MNG datastream, decoders can improve
the speed and consistency of the display.
For single-frame MNG datastreams, however, decoding speed is less
important and exact color rendition might be more important. Therefore, it
is best to leave the images in their original color space, as recommended in
the PNG specification, retaining the individual color space chunks if the
images have different color spaces. This will avoid any loss of data due
to conversion.
10.2. Use the right framing mode
Always use framing mode 1 or 2 when all of the images are opaque.
This avoids unnecessary screen clearing, which can cause flickering.
10.3. Immediate frame sync point
If it
is necessary to establish a synchronization point immediately, this can
be done by using two consecutive
FRAM
chunks, the first setting
a temporary
interframe_delay=0
timeout
, and
sync_id
, and the second establishing the synchronization
point:
FRAM 2 0 1 1 0 1 0000 timeout sync_id
FRAM 0 name
10.4. Embedded images in LOOPs
Embedded images should not be enclosed in loops unless absolutely
necessary. It is better to store them ahead of time and then use
SHOW
chunks inside the loops. Otherwise, decoders will be
forced to repeatedly decode them. See Examples 2, 8, 11, and 12,
below (Chapter 18)
10.5. Including optional index in SAVE chunk
Authors of MNG files that are intended for transmission over a
network should consider whether it is more economical for the client to
rebuild the index from scratch than it is to transmit it. Web pages
that are likely to be downloaded over slow lines, and whose clients
are unlikely to use the index anyway, generally should have empty
chunks. No information is lost by deleting the index,
because the MNG datastream contains all of the information needed to
build the index. If an application does build an index, and the file
is going to be kept as a local file, the application should replace
the empty
chunk with one containing the index. See
above (Paragraph 4.4.1)
10.6. Interleaving JDAT, JDAA, and IDAT chunks
When a JNG datastream contains an alpha channel, and the file is
intended for transmission over a network, it is useful to interleave
the
IDAT
or
JDAA
and the
JDAT
chunks.
In the case of sequential
JPEG, the interleaving should be arranged so that the alpha data
arrives more or less in sync with the color data for the scanlines.
In the case of progressive JPEG, the alpha data should be interleaved
with the first JPEG pass, so that
all
of the alpha data has
arrived before the beginning of the second JPEG pass.
10.7. Use of the JDAA chunk
It is recommended that the
JDAA
chunk be used only to convey
smoothly varying alpha channels and not to convey binary transparency
which is more precisely and efficiently conveyed in
IDAT
chunks.
11. Recommendations for Decoders
11.1. Using the simplicity profile
The simplicity profile in the
MHDR
chunk can be ignored or
it can be used for
Deciding whether to abandon a datatream immediately if it is beyond
the decoder's capabilities. Decoders are of course free to
plunge ahead, rendering whatever is possible and abandoning any segments
that contain critical chunks that they do not recognize or cannot handle.
Unmanageable features might not be present even when the simplicity
profile indicates that the features "might be present".
The profile
never guarantees that a certain feature is present; it only guarantees
that certain features are not present or have no effect on the appearance
of any frame.
Deciding whether to perform certain optimizations. For example, the
transparency flags can be used to determine whether full alpha composition
is going to be necessary, and to choose appropriate code paths and
internal representations of abstract objects accordingly.
11.2. ENDL without matching LOOP
If a decoder reads an
ENDL
chunk for which the matching
LOOP
chunk is missing, or has been skipped for some reason, any
active loops with a higher
nest_level
should be terminated,
and processing can resume after the next
SEEK
chunk. Simple
viewers that do not process the
chunk should abandon the
MNG datastream. See
above
11.3. Note on compositing
The PNG specification gives a good explanation of how to composite a
partially transparent image over an opaque image, but things get more
complicated when both images are partially transparent.
Pixels in PNG and JNG images are represented using gamma-encoded RGB
(or gray) samples along with a linear alpha value. Alpha processing
can only be performed on linear samples. This chapter assumes that R,
G, B, and A values have all been converted to real numbers in the range
[0..1], and that any gamma encoding has been undone.
For a top pixel {Rt,Gt,Bt,At} and a bottom pixel {Rb,Gb,Bb,Ab}, the
composite pixel {Rc,Gc,Bc,Ac} is given by:
Ac = 1 - (1 - At)(1 - Ab)
if (Ac != 0) then
s = At / Ac
t = (1 - At) Ab / Ac
else
s = 0.0
t = 1.0
endif
Rc = s Rt + t Rb
Gc = s Gt + t Gb
Bc = s Bt + t Bb
When the bottom pixel is fully opaque (Ab = 1.0), the function
reduces to:
Ac = 1
Rc = At Rt + (1 - At) Rb
Gc = At Gt + (1 - At) Gb
Bc = At Bt + (1 - At) Bb
When the bottom pixel is not fully opaque, the function is
much simpler if premultiplied alpha is used. A pixel that uses
non-premultiplied alpha can be converted to premultiplied alpha by
multiplying R, G, and B by A.
For a premultiplied top pixel {Rt,Gt,Bt,At} and a premultiplied
bottom pixel {Rb,Gb,Bb,Ab}, the premultiplied composite pixel
{Rc,Gc,Bc,Ac} is given by:
Ac = 1 - (1 - At)(1 - Ab)
Rc = Rt + (1 - At) Rb
Gc = Gt + (1 - At) Gb
Bc = Bt + (1 - At) Bb
As mentioned in the PNG specification, the equations become much
simpler when no pixel has an alpha value other than 0.0 or 1.0, and the
RGB samples need not be linear in that case.
11.4. Retaining object data
The decoder must retain information about each object (except for
objects with
object_id=0
) for possible redisplay with the
SHOW
chunk or for possible use as the parent object for a
subsequent Delta-PNG datastream.
The following information must be retained, for each nonzero object
that is defined and not subsequently discarded:
The set of object attributes (potential visibility, location, clipping
boundary data from the
DEFI
MOVE
CLIP
, and
SHOW
chunks, and pointer to an object buffer).
The pixel data and the values associated with other recognized PNG
chunks such as
PLTE
and
gAMA
, subject to the chunk
copying rules and the
DROP/DBYK
chunks. If the object is
"abstract", the data can be stored in any convenient form. If it is
"concrete", it must be stored in an object buffer in a manner that would
permit the complete restoration of the original PNG or JNG file or its
equivalent.
The most recent values of
target_x
and
target_y
, if the object was the destination of a
PAST
chunk.
When the encoder knows that data in the object buffer will not be
needed later, it help decoders operate more efficiently by
using
object_id=0
or by using the
DISC
or the
SEEK
chunk. Abstract images rather than concrete objects
should be used if the encoder knows that the data will not later be used
as the parent object for a Delta-PNG. If no object buffer in the entire
datastream will be needed later, the "stored object buffers" flag
can be set appropriately in the simplicity profile field of the
MHDR
chunk.
11.5. Decoder handling of fatal errors
When a fatal error is encountered, such as a bad CRC or an unknown
critical MNG chunk, minimal viewers
that do not implement the
SAVE/SEEK
mechanism
should simply abandon the MNG datastream.
More capable MNG viewers should attempt to recover gracefully by
abandoning processing of the segment and searching for a
SEEK
chunk. If such errors occur before the
chunk is reached,
the viewer should abandon the MNG datastream.
When an error occurs within a image datastream, such as an unknown
critical PNG chunk or a missing parent object where one was required,
only that image should be abandoned and the associated object should be
discarded. If a bad CRC is found, indicating a corrupted datastream,
the entire segment should be abandoned, as above.
MNG editors, on the other hand, should be more strict and reject any
datastream with errors unless the user intervenes.
11.6. Decoder handling of interlaced images
Decoders are required to be able to interpret datastreams that
contain interlaced PNG images, but are only required to display the
completed frames; they are not required to display the images as they
evolve. Viewers that are decoding datastreams coming in over a slow
communication link might want to do that, but MNG authors should not
assume that the frames will be displayed in other than their final form.
11.7. Decoder handling of palettes
When a
PLTE
chunk is received, it only affects the display
of the PNG datastream that includes
or inherits
it. Decoders must
take care that it does not retroactively affect anything that has already
been decoded.
If
PLTE
or
PPLT
is present in a Delta-PNG
datastream, the new palette is used in displaying the image defined by
the Delta-PNG; if no
IDAT
chunk is present and the image type
is PNG indexed-color, then the resulting image is displayed using the
old pixel samples as indices into the new palette, which provides a
"palette animation" capability.
If a frame contains two or more images, the
PLTE
chunk in
one image does not affect the display of the
other, unless one image
is a subsequent Delta-PNG that has no
PLTE
chunk and has been
declared by the
DHDR
object_id
field to depend on the
other.
A composite frame consisting only of indexed-color images should not
be assumed to contain 256 or fewer colors, since the individual palettes
do not necessarily contain the same set of colors.
Encoders can supply
a top-level
sPLT
chunk with a suggested reduced global palette,
to help decoders build an appropriate palette when necessary.
11.8. Behavior of single-frame viewers
Viewers that can only display a single frame must display the first
frame that they encounter.
It is strongly recommended that such viewers
understand the
fPRI
chunk
above (Paragraph 4.5.2)
which
will enable them to find and display the best representative frame, if
the encoder has identified one.
11.9. Clipping
MNG provides four
types of clipping, in addition to any clipping that
might be required due to the physical limitations of the display device.
Frame width and frame height
The
frame_width
and
frame_height
are defined in
the
MHDR
chunk and cannot be changed by any other MNG chunk.
This is the only type of clipping available in MNG-VLC
datastreams.
Decoders can use these parameters to establish the size of
a window in which to display the MNG frames. When the
frame_width
or
frame_height
exceeds the physical dimensions of the
display hardware, the contents of the area outside those dimensions is
undefined. If a viewer chooses, it can create "scroll bars" or the
like, to enable persons to pan and scroll to the offscreen portion
of the frame. If this is done, then the viewer is responsible for
maintaining and updating the offscreen portion of the frame.
In the case of a MNG datastream that consists of a PNG or JNG
datastream, with the PNG or JNG signature, the
frame_width
and
frame_height
are defined by the
width
and
height
fields of the
IHDR
(or
JHDR
) chunk.
Layer clipping boundaries
The layer clipping boundaries are optionally defined in the
FRAM
chunk, and cannot be changed within a subframe. When
the framing mode is 3 or 4, viewers must, prior to displaying
the foreground layers of each frame, clear the area within the layer
clipping boundaries to the background color,
and display the background image if one has
been defined,
thus creating a separate layer at the beginning of each frame.
Viewers must not change any pixels outside the layer
boundaries; encoders must be able to rely on the fact that the part of
the display that is outside the layer clipping boundaries (but inside
the area defined by
frame_width
and
frame_height
will remain on the display from frame to frame without being explicitly
redisplayed.
See
Example 8
which displays a
large background image once, and then, in each frame, only redisplays
the portion of the background surrounding the moving sprite.
Image clipping boundaries
The image clipping boundaries are defined in the
DEFI
and
CLIP
chunks.
They are associated with individual objects, not
with the layers, and they can be changed within a subframe of layers.
They are useful for exposing only a portion of an image in a
frame, to achieve
effects such as scrolling, panning, or gradual exposure.
The clipping boundaries are expressed in pixels, measured rightward
and downward from the frame origin.
The left and top clipping boundaries are inclusive and the right and
bottom clipping boundaries are exclusive, i.e., the pixel located at
{x,y} is only displayed if the pixel falls within the physical limits of
the display hardware and all of the following are true:
0 <= x < frame_width (from the MHDR chunk)
0 <= y < frame_height
Left_lcb <= x < right_lcb (from the FRAM chunk)
Top_lcb <= y < bottom_lcb
Left_cb <= x < right_cb (from the DEFI or CLIP chunk)
Top_cb <= y < bottom_cb
PAST
clipping boundaries
One type of clipping performed in
PAST
gives a fourth type that has
no dependencies on the other types, since the object
CLIP
data
is ignored and the
PAST
chunk defines its own clipping boundaries
within the destination object.
The left and top of this type of clipping is also inclusive, and the
right and bottom are exclusive.
Clipping to
PAST
destination object dimensions
A second type of clipping performed in
PAST
gives a fifth
type that also has no dependencies on the other types. The result of all
PAST
operations is clipped to fall within the dimensions of the
destination object.
The left and top of this type of clipping is also inclusive, and the
right and bottom are exclusive. After this internal clipping is completed,
the destination object is clipped in the same manner as other objects
when it is displayed.
12. Recommendations for Editors
12.1. Editing datastreams with optional index
Editors must recreate or delete the optional
chunk
index whenever they make any change that affects the offsets of chunks
following the portion of the datastream that is changed. If the changes
do not involve the addition, deletion, or relocation of segments,
frames, and images, then it is sufficient to zero out the offsets.
The
chunk is not considered to be in any MNG segment,
so changing it has no effect on the copy-safe status of unknown chunks
in any other part of the MNG datastream.
When the
chunk is expanded to include an index, all
chunks that follow will have their offsets changed by an amount equal
to the change in the length of the data segment of the
chunk, so the offset table will have to be adjusted accordingly. If a
chunk is already present with zero offsets, the correct
offsets can be written without adjustment.
12.2. Handling LOOP and TERM chunks
Editors that create a series of PNG or JNG datastreams from a MNG datastream
should check the termination condition of any
LOOP
chunks and execute
loops only
iteration_min
times. The loop created by the
TERM
chunk should be executed only once.
13. Miscellaneous Topics
13.1. File name extension
On systems where file names customarily include an extension
signifying file type, the extension
.mng
is recommended for
MNG (including MNG-LC and MNG-VLC)
files. Lowercase
.mng
is
preferred if file names are case-sensitive. The extension
.jng
is
recommended for JNG files.
13.2. Internet media type
When and if the MNG format becomes finalized, the
MNG authors intend to register
video/mng
as the Internet Media Type for
MNG
RFC-2045
RFC-2048
At the date of this document, the media
type registration process had not been started. It is recommended
that implementations also recognize the interim media type
video/x-mng
Although we define a standalone JNG format, we recommend that such
files be used only temporarily while compiling or disassembling MNG
datastreams. We may at some future time register an Internet Media
Type for JNG files. Until then, the interim media type
image/x-jng
can be used.
13.3. EBNF Grammar for MNG, PNG, and JNG
An Extended Backus-Naur Form (EBNF) description of the chunk ordering
in MNG, PNG, and JNG is being developed. The
current draft, together with some supporting software, is available at
ftp://swrinde.nde.swri.edu/pub/mng/documents/ebnf/
13.4. Uniform Resource Identifier (URI)
Segments, subframes, and objects are externally accessible via named
SEEK
eXPI
, and
FRAM
chunk names. They can be
referred to by URI, as in
SRC=file.mng#segment_name
SRC=file.mng#subframe_name
SRC=file.mng#snapshot_name
SRC=file.mng?segment_name#segment_name
SRC=file.mng?snapshot_name#snapshot_name
When the URI specializer ("#" or "?")
is "#", and the fragment
identifier (the string following the specializer) is the name of a
segment, i.e., a named
SEEK
chunk, the viewer should display
the sequence from the beginning of the named segment up to the next
segment. When it refers to a subframe or an image, i.e., a named
FRAM
or
eXPI
chunk, it should display the single frame
(as it exists when the next
FRAM
chunk is encountered) or image
that is identified by the fragment identifier.
The client can find the needed segment quickly if the
chunk is present and contains the optional index.
When the URI specializer is "?" (server side query),
the "query
component" is the string following the "?" specializer
and up to but not including the "#" if
the "#" specializer is also present. The
server should find the segment that is named in the query component
or the segment that contains the subframe or image named in the query
component, and it should return a datastream consisting of:
all chunks prior to the
chunk,
an empty
chunk,
the
SEEK
chunk for the segment being returned,
all chunks in the segment, and
MEND
chunk.
If no
chunk is present, the server must simply return
the entire MNG datastream. Servers that are unwilling to parse the MNG
datastream and are unconcerned about bandwidth can return the entire MNG
datastream even when the SAVE chunk is present. Authors should defend
against this behavior by including both a query and a fragment in the
URI even when a segment is being requested.
The client can process this as a complete MNG datastream, either
displaying the entire segment, if no fragment identifier is present, or
extracting the segment, frame or image that is named in a fragment
identifier and displaying it, if a fragment identifier is present
(a fragment identifier must be present if a frame or image is being
requested). To "extract a frame" means to decode
the returned datastream through the end of the frame that contains the
named subframe and to display the result as a single still image. If
the layers of the named subframe do not cover the entire frame, pixels
from the background and from earlier subframes must be included in the
resulting composition.
A part of the MNG datastream can also be requested by timecode, as in
SRC=file.mng#clock(10s-20s)
SRC=file.mng#clock(0:00-0:15)
SRC=file.mng?clock(0:00-0:15)#clock(0:00-0:15)
or by frame number, as in
SRC=file.mng#frame(10)
SRC=file.mng#frames(30-60)
SRC=file.mng?frames(30-60)#frames(30-60)
The timecode must consist of starting and ending clock values, as
defined in the W3C SMIL recommendation, separated by a hyphen (ASCII
code 45).
When the URI specializer is "#", the viewer should play
that part of
the sequence beginning and ending at the requested times, measuring
from zero time at the beginning of the MNG datastream, or beginning and
ending with the specified frame numbers. To do this it must start with
the segment containing the requested time and decode any part of the
segment up to that time, composing but not displaying the frames; this
will provide the background against which the desired frames are
displayed.
When the URI specializer is "?", the server can send the
entire MNG datastream, or, preferably, it should construct a complete MNG file
containing:
the chunks preceding the
chunk,
the
chunk itself with an optional index that gives
the starting time and starting frame number of the first
SEEK
chunk
that is sent, and
the one or more consecutive sets of segments, with their
SEEK
chunks, that contain the sequence beginning and ending at
the requested times, or frame numbers, at the proper framing rate.
If the server does not send the entire MNG datastream, and the first
segment after the
chunk is not sent but a later segment
is
sent, the optional index
must be written even if it does not exist in the source file. The index
must contain at least one "type 0" entry that gives the nominal
start
time and frame number for the first segment that is sent after the
chunk. The offset field can be set to zero and the segment
name can be omitted.
The query component should always be repeated as a fragment
identifier, so clients can find the requested item in case the server
sends more than what was requested.
MNG datastreams should not contain segment, subframe, or image
names that begin with the case-insensitive strings
"CLOCK(", "FRAME(", or "FRAMES(",
which are reserved for use in URI queries and
fragments (see Uniform Resource Identifier
below
).
See
RFC-2396
and the W3C SMIL recommendation
at
14. Rationale
This (incomplete as of version 1.0) section does not form a part of
the specification. It provides
the rationale behind some of the design decisions in
MNG.
Interframe delay
Explain why the interframe delay has to be provided
before
the subframes of layers are defined, instead of having a simpler
DELA
chunk that occurs in the stream where the delay is
wanted.
DHDR delta types
Some delta types are not allowed when the parent object is a JNG
image. Explain why types 4 and 6 (pixel replacement and color channel
replacement) are not allowed under these circumstances.
Additional filter methods
Filter method 64 could have been implemented as a new critical chunk in
embedded PNG datastreams.
FILT
method (1 byte)
64: intrapixel differencing
data (variable, depends on method)
method 64 requires no data
The
FILT
chunk would turn on this type of filtering.
The choice of using a new filter method instead of a new critical chunk was
made based on simplicity of implementation and possible eventual inclusion
of this method in PNG. Also, using the filter-method byte helps implementors
avoid confusion about whether this is a color transform (which could affect
the implementation of
tRNS
and other color-related chunks) or part
of the filtering mechanism (which would not conceivably affect color-related
chunks).
We considered using an ancillary chunk (e.g.,
fILt
or
fILT
) to turn on the new
filtering method. This would have the advantage that existing applications
could manipulate the files, but viewers that ignore the chunk would display
the image in unacceptably wrong colors, and editors could mistakenly discard
the chunk.
MAGN chunk rationale
Q. Why not just use a
BASI
chunk to encode solid-color rectangles?
A. The
MAGN
chunk also allows encoding of gradient-filled
rectangles.
Q. Why not just use PNG to encode gradient-filled rectangles?
A. While PNG can encode vertical and horizontal gradients fairly
efficiently, it cannot do diagonal ones efficiently, and none
are as efficient as a 30-byte MAGN chunk plus a 4-pixel PNG.
Q. Why not use full-scale low-quality JPEG/JNG?
A. Low-quality JPEG with reduced dimensions can be much smaller than even
the lowest-quality full-sized JPEG. Such images can then be magnified to
full scale with the
MAGN
chunk, for
use as preview ("LOWSRC") images.
this has been demonstrated to be about 40 to 50 times as efficient as using
Adam7 interlacing of typical natural images,
It appears that in general, usable preview images of truecolor
photographic images can be made at compression ratios from M*800:1 to
M*2500:1, where M is the number of megapixels in the original image, by
reducing the original image spatially to width and height in the range
64 to 200 pixels and then compressing the result to a medium-quality JNG.
Q. Why not use the
pHYg
chunk?
A. It is not mandatory for decoders to process the
pHYg
chunk
and it does not apply to individual images; it
is used to scale the entire MNG frame. The
pHYs
chunk cannot be
used either because MNG decoders are required to ignore it.
Q. Why not 4-byte magnification factors instead of 2-byte ones?
A. Encoders can start with a larger object or, except for
object 0, magnify it twice.
Q. Why not 1-byte magnification factors, then?
A. With typical screen widths currently 1280 or 1600 pixels and film and
printer pages currently about 3000 pixels wide, magnifying a 1x1 image
to a width of more than 255 pixels would not be uncommon.
Q. I want to magnify a "frozen" object.
A. You can make a full clone and magnify that.
Q. Why define Methods 4 and 5?
A. Method 4 is useful for magnifying an alpha-encoded image while maintaining
binary transparency. Method 5 is useful for making an alpha-gradient
while preserving sharp edges in the main image.
Global JPEG tables
It has been suggested that a new global MNG chunk,
JTAB
be defined to hold global JPEG quantization and Huffman tables that
could be inherited by JNG datastreams from which these have been
omitted. This has not been tested, and we are reluctant to add new
critical chunks to the MNG specification now.
15. Revision History
15.1. Version 1.0
Released 31 January 2001
Changed the meaning of the
FRAM
timeout. Instead of being
added to the interframe delay, it is a minimum or maximum value to which
the decoder can change the interframe delay. This was approved by
consensus on December 23, 2000.
Added a section on Extension and Registration.
Minor editorial and formatting changes.
15.2. Version 0.99
Released 10 December 2000
Miscellaneous technical changes
A new filter method (method 64, intrapixel differencing) is defined for
PNG datastreams that are embedded in MNG-LC, MNG, and
Delta-PNG datastreams. This was approved by formal vote on December 4, 2000.
Deleted "or can be ignored" from the definition of the background
transparency profile flag.
This was approved by consensus on October 28, 2000.
Revised definition of magnification methods 3 and 4 and added method
5 for the
MAGN
chunk.
This was approved by consensus on November 11, 2000.
Clarified that "saved" data need only be restored when a
decoder makes random access to a seek point after jumping from the interior
of a segment.
This was approved by consensus on October 28, 2000.
Clarified that the background image must be potentially visible to
be displayed.
This was approved by consensus on October 28, 2000.
When the sample depth in a delta-PNG is larger than the sample depth
of the parent object, right-shifting of the delta is specified.
This was approved by consensus on November 10, 2000.
Clarified that the
MAGN
chunk can generate one or more
layers, when the existing objects being magnified are potentially
visible.
Added a security recommendation to check for user input after each
loop iteration as well as after each complete frame, to avoid being stuck
in an infinite loop of subframes with zero interframe delay.
Clarifications
Added the
MAGN
and
CLON
chunks to the list of chunks
across which MNG editors cannot move unknown ancillary chunks.
Added "foreground layer" terminology.
Editorial changes to the FRAM chunk specification to clarify when
interframe delays and synchronization points occur.
Added paragraph about object 0 in the introductory section on objects.
Clarified that object attributes for object 0 become undefined
when a
SEEK
chunk appears, if they are different from the
default values at the end of any segment. Made the treatment of magnification
data for object 0 consistent with the treatment of the other attributes.
Removed statement in the
FRAM
chunk specification that a
subframe ends when a
SEEK
chunk is encountered. This is inconsistent
with statements elsewhere in the specification that the
SEEK
chunk
can be treated as if it were an empty
DISC
chunk.
Clarified that inserting a background layer ahead of a segment is only
necessary when the decoder jumps to a seek point from the interior of a
segment.
Clarified that the empty
DISC
chunk only discards nonzero objects.
Revised the author list.
15.3. Version 0.98
Released 01 October 2000
Added JPEG-encoded alpha channel in JNG and Delta-PNG datastreams,
stored in a new
JDAA
chunk. This was approved by a formal vote.
Added the
MAGN
chunk. This was approved by a formal vote.
Caution: there were errors in the interpolation formula
for
MAGN
(unbalanced parentheses, "+m" was
"+1") in the proposal that was voted upon; those errors have
been fixed in this public release.
Added a "stored object buffers" flag to promise that even
when "complex MNG features" are present, it is not necessary
to create object buffers. This proposal was approved by a formal vote.
Separated the "transparency" profile bit
into "transparency", "semitransparency",
and "background transparency", and added discussion
of "background
transparency" to the BACK
and FRAM chunk specifications.
This
proposal was approved by a formal vote.
Added a "validity" flag to maintain backward compatibility
of the simplicity profile. If it is zero, then the "background
transparency", "semitransparency",
and "stored object buffers" flags do
not
make any promises.
Global
sRGB
nullifies global
gAMA
and
cHRM
and
vice versa
It is permitted to change the potential visibility,
location, and clipping boundaries of "frozen" objects, provided
that the encoder writes chunks to restore them to their "frozen"
values prior to the end of the segment.
Added a note that top-level color-space chunks do not have any effect
on already-decoded objects.
Mentioned a fifth type of clipping: clipping the result of
PAST
operations to the dimensions of the
PAST
destination object.
Disallowed the
JSEP
chunk when
image_sample_depth != 20
Clarified some wording in the
SEEK
chunk specification, and
added a cross reference to the existing requirement to insert a background
layer when making random access to a segment.
Added terminology entries for "animation",
"framing rate",
"interpolation",
"iteration",
"replication",
and "nullify".
Clarified treatment of the alpha sample in the
BASI
chunk
when the color type is 0, 2, or 3, and clarified that the
BASI
chunk inherits default
DEFI
values if no
DEFI
chunk
is present.
Changed "repeat count" to "iteration count" in
the
LOOP
chunk specification, and "times to
repeat" to "times to execute" in the description of
the "iteration_max" field in the TERM chunk, and
added a statement about representing infinity.
Added two examples related to the
MAGN
chunk.
Various editorial changes.
15.4. Version 0.97
Released 28 February 2000.
Minor editorial changes only.
A new example was added.
15.5. Version 0.96
Released 18 July 1999.
The changes that are not simple editorial
changes were approved by votes of the
PNG Development group that closed 16 July 1999 (
pHYg
and change to treatment of the
pHYs
chunk), 14 July 1999 (global
bKGD
and
sBIT
) and 25 June 1999 (change to
LOOP
chunk and treatment of the
DEFI
chunk and nonviewable objects).
An object "comes into existence" when it is named in a
DEFI
chunk instead of later, when the corresponding embedded image is
received.
This makes it possible to
MOVE
or
CLIP
objects whose
object buffer does not yet exist.
The special treatment of the set of object attributes for object 0 was
eliminated.
Any attempt to display a nonviewable object must be ignored and not
treated as an error. The restriction that a nonviewable object must
not be made potentially visible was removed.
Any nonviewable object included in the list of
objects to be processed by the
SHOW
chunk must be ignored and
not treated as an error (in MNG-0.95 and earlier, the
SHOW
chunk
would change its visibility but not display it).
If fields are omitted from the
DEFI
chunk, values are inherited
from a previous
DEFI
chunk, if one was present. In MNG-0.95,
such fields assumed specified default values. In this version, the
default values are only used if no prior
DEFI
chunk with the same
object_id was present or if the prior
DEFI
chunk has been discarded.
The
termination_condition
byte of the
LOOP
chunk was
extended to include a "cacheable" bit.
Revised wording of paragraph 3.3 to describe "viewable
objects" as well as "viewable object buffers".
Clarified that an image is displayed immediately if it is the
subject of a
CLON
chunk with
do_not_show=0
Revised Examples 6, 7, 9, 13 and 14.
Changed "
JDAT_sample_depth
" to
image_sample_depth
" and
IDAT_sample_depth
" to
alpha_sample_depth
", etc.
Started a Rationale section.
Started a Revision History section.
Added the
pHYg
chunk and changed
the meaning of the global
pHYs
chunk.
Added the global
bKGD
and
sBIT
chunks.
15.6. Version 0.95
Initial public release, approved by the PNG Development Group
on 11 May 1999.
16. References
ISO/IEC-10918-1
International Organization for Standardization and International
Electrotechnical Commission,
"Digital Compression and Coding of Continuous-tone Still Images,
Part 1: Requirements and guidelines" ISO/IEC IS 10918-1, ITU-T T.81.
See also
Pennebaker, William B., and Joan L. Mitchell,
"JPEG : Still Image Data Compression Standard"
Van Nostrand Reinhold, ISBN:0442012721, September 1992
JFIF
C-Cube Microsystems,
"JPEG File Interchange Format, Version 1.02",
September 1992.
LOCO
Weinberger, Marcelo J., Gadiel Seroussi, and Guillermo Sapiro, "The
LOCO-I Lossless Image Compression Algorithm: Principles and Standardization
into JPEG-LS" Hewlett Packard Report HPL-98-193R1, November 1998, revised
October 1999, available at
PNG
Boutell, T., et. al., "PNG (Portable Network Graphics Format)
Version 1.0", RFC 2083,
ftp://ftp.isi.edu/in-notes/rfc2083.txt
also available at
ftp://swrinde.nde.swri.edu/pub/png/documents/
This
specification has also been published as a W3C Recommendation, which is
available at
See also the PNG-1.2 specification:
Randers-Pehrson, G., et. al., "PNG (Portable Network Graphics
Format) Version 1.2", which is available at
ftp://swrinde.nde.swri.edu/pub/png/documents/
PNG-EXT
Randers-Pehrson, G., et al,
"Extensions to the PNG 1.2 Specification",
ftp://swrinde.nde.swri.edu/pub/png/documents/pngext-*
[RFC-2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119/BCP 14, Harvard University, March 1997.
[RFC-2045]
Freed, N., and N. Borenstein, "Multipurpose Internet Mail Extensions
(MIME) Part One: Format of Internet Message Bodies", RFC 2045, Innosoft,
First Virtual, November 1996.
ftp://ftp.isi.edu/in-notes/rfc2045.txt
[RFC-2048]
Freed, N., Klensin, J., and J. Postel, "Multipurpose Internet Mail
Extensions (MIME) Part Four: Registration Procedures", RFC 2048,
Innosoft, MCI, USC/Information Sciences Institute, November 1996.
ftp://ftp.isi.edu/in-notes/rfc2048.txt
[RFC-2396]
Berners-Lee, T., R. Fielding, U. C. Irvine, and L. Masinter,
"Uniform Resource
Identifiers (URI): Generic Syntax", RFC 2396, MIT/LCS,
Xerox Corporation,
University of Minnesota, August 1998.
ftp://ftp.isi.edu/in-notes/rfc2396.txt
17. Security Considerations
Security considerations are addressed in the PNG specification.
An infinite or just overly long loop could give the appearance
of having locked up the machine, as could an unreasonably long
interframe delay or a misplaced
sync_id
with a long
timeout
value. Therefore a decoder should always
provide a simple method for users to escape out of a loop or delay,
either by abandoning the MNG entirely or just proceeding to the next
SEEK
chunk. Decoders should check for user input after
each loop iteration (not just after each frame) in case of infinite loops
that are empty or that generate layers with zero interframe delay.
The
SEEK
chunk makes it safe for a
viewer to resume processing after it encounters a corrupted portion of a
MNG datastream or jumps out of the interior of a segment for any reason.
Some people may experience epileptic seizures when they are exposed
to certain kinds of flashing lights or patterns that are common in
everyday life. This can happen even if the person has never had any
epileptic seizures. All graphics software and file formats that
support animation and/or color cycling make it possible to encode
effects that may induce an epileptic seizure in these individuals.
It is the responsibility of authors and software publishers to issue
appropriate warnings to the public in general and to animation creators
in particular.
No known additional security concerns are raised by this format.
18. Appendix: Examples
We use the "#" character to denote commentary in these
examples; such comments are not present in actual MNG datastreams.
18.1. Example 1: A single image
The simplest MNG datastream is a single-image PNG datastream. The
simplest way to create a MNG from a PNG is:
copy file.png file.mng
The resulting MNG file looks like:
\211 P N G \r \n ^z \n # PNG signature.
IHDR 720 468 8 0 0 0 0 # Width and Height, etc.
sRGB 2
gAMA 45455
IDAT ...
IEND
If
file.png
contains an
sRGB
chunk and
also
gAMA
and
cHRM
chunks that are recommended in the
PNG specification for "fallback" purposes, you can remove
those
gAMA
and
cHRM
chunks from
file.mng
because any MNG viewer that processes the
gAMA
chunk is also
required to recognize and process the
sRGB
chunk, so those
chunks will always be ignored. Any MNG editor that converts the MNG file
back to a PNG file is supposed to insert the recommended
gAMA
and
cHRM
chunks.
18.2. Example 2: A very simple movie
This example demonstrates a very simple movie, such as might result
from directly converting an animated GIF that contains a simple series
of full-frame images:
\212 M N G \r \n ^z \n # MNG signature.
MHDR 256 300 # Width and height.
1 # 1 tick per second.
5 4 4 # Layers, frames, play time
583 # Simplicity profile
DEFI 1 0 0 IHDR ... IDAT ... IEND # Four PNG datastreams
DEFI 2 0 0 IHDR ... IDAT ... IEND # are read and stored
DEFI 3 0 0 IHDR ... IDAT ... IEND # and are displayed as
DEFI 4 0 0 IHDR ... IDAT ... IEND # they are read.
SAVE # This is needed so we can place TERM before SEEK.
TERM 3 0 120 10 # When done, repeat from TERM 10 times.
SEEK
SHOW
MEND
18.3. Example 3: A simple slideshow
\212 M N G \r \n ^z \n # MNG signature.
MHDR 720 468 1 # Width and height, 1 tick per second.
6 5 5 # Layers, frames, play time.
67 # Simplicity profile (MNG-LC no transparency)
FRAM 1 0 2 2 0 2 1 600 0 # Set interframe_delay to 1,
# timeout to 600 sec, and sync_id list to {0}.
SEEK "Briefing to the Workforce"
IHDR ... IDAT ... IEND # DEFI 0, visible, abstract
SEEK "Outline" # is implied.
IHDR ... IDAT ... IEND
SEEK "Our Vision" IHDR ... IDAT ... IEND
SEEK "Our Mission" IHDR ... IDAT ... IEND
SEEK "Downsizing Plans" IHDR ... IDAT ... IEND
MEND
18.4. Example 4: A more storage-efficient slideshow
This slideshow gives exactly the same output as Example 3, but the
storage in the datastream is more efficient (the IDAT chunks will be
smaller) while the memory requirements in the decoder are larger. Image
ID 1 is used to store the ornate logos and frame design that appear on
every slide. The DHDR-IEND datastreams only contain deltas due to the
text and other information that is unique to each slide.
\212 M N G \r \n ^z \n # MNG signature.
MHDR 720 468 # Width and height.
1 6 5 5 975 # 1 tick per second, complex, no JNG.
DEFI 1 1 1 # Define image 1, invisible, concrete.
IHDR ... IDAT ... IEND
FRAM 1 0 2 2 0 2 1 600 0 # set interframe_delay to 1,
# timeout to 600 sec and sync_id list to {0}.
SEEK "Briefing to the Workforce"
CLON 1 2 DHDR 2 ... IDAT ... IEND SHOW 2
SEEK "Outline"
CLON 1 2 DHDR 2 ... IDAT ... IEND SHOW 2
SEEK "Our Vision"
CLON 1 2 DHDR 2 ... IDAT ... IEND SHOW 2
SEEK "Our Mission"
CLON 1 2 DHDR 2 ... IDAT ... IEND SHOW 2
SEEK "Downsizing Plans"
CLON 1 2 DHDR 2 ... IDAT ... IEND SHOW 2
MEND
18.5. Example 5: A simple movie
This movie is still fairly simple, but it capitalizes on
frame-to-frame similarities by use of Delta-PNG datastreams, and also
demonstrates the use of the
fPRI
chunk.
\212 M N G \r \n ^z \n # MNG signature.
MHDR 720 468 # Width and height.
30 6 5 15 # 30 ticks per second.
975 # Delta-PNG, transparent, complex
tEXtTitle\0Sample Movie
fPRI 0 128 # Default frame priority is "medium".
FRAM 1 0 2 0 0 0 3 # Set interframe_delay to 1/10 sec.
DEFI 1 0 1 # Set default image to 1 (concrete).
SEEK "start"
IHDR 720 468 8 2 0 0 0 # DEFI 1 is implied.
IDAT ...
IEND
DHDR 1 1 1 20 30 100 220 # A PNG-delta frame.
IDAT ... # The IDAT gives the 20x30 block
IEND # of deltas.
DHDR 1 1 1 20 30 102 222 # Another PNG-delta frame.
IDAT ... # This time the deltas are in a 20 x 30
IEND # block at a slightly different location.
SEEK "frame 3" # OK to restart here because a
# complete PNG frame follows.
fPRI 0 255 # This is the representative frame that
IHDR 720 468 ...# will be displayed by single-frame
IDAT ... # viewers.
IEND
fPRI 0 128 # Return to medium frame priority.
DHDR 1 1 1 720 468 0 0 # Another PNG-delta frame.
IDAT ... # The entire 720x468 rectangle changes
IEND # this time.
SEEK "end"
MEND # End of MNG datastream.
18.6. Example 6: A single composite frame
Here is an example single-composite-frame MNG, with thumbnails, which
takes a grayscale image and draws it side-by-side with a false-color
version of the same image:
\212 M N G \r \n ^z \n # MNG signature.
MHDR 1024 512 0 # Width, height, ticks per second
4 1 0 1007 # Layers, frames, time, simplicity
BACK 16448 16448 52800 1 # Must use sky blue background.
PLTE ... # Define global PLTE
gAMA 50000 # Define global gAMA
DEFI 1 1 # Define invisible abstract thumbnail image.
IHDR 64 64 4 3 0 0 0 PLTE IDAT ... IEND # use global PLTE
eXPI 1 "thumbnail 1"
DEFI 1 1 # Also define a larger thumbnail.
IHDR 96 96 4 3 0 0 0 PLTE IDAT ... IEND # use global PLTE
eXPI 1 "thumbnail 2"
DISC # Discard the thumbnail image.
FRAM 4 "Two views of the data"
DEFI 1 0 1 6 6 # Define first (bottom) image.
IHDR 500 500 16 0 .. # A 16-bit graylevel image.
IDAT ...
IEND # End of image.
CLON 1 2 0 1 0 0 518 6 # Make full invisible concrete clone.
SHOW 2 2 3 # Mark it for immediate display during
# the upcoming delta-PNG operation.
DHDR 2 1 7 # Modify it (no change to pixels).
ORDR faLT 2 # Establish chunk placement.
gAMA 100000 # Local gamma value is 100000 (gamma=1.0).
tEXtComment\0The faLT chunk is described in ftp://swrinde...
faLT ... # Apply pseudocolor to parent image.
IEND # End of image.
DEFI 3 0 0 900 400 # Overlay near lower right-hand corner.
IHDR 101 101 2 3 ...
PLTE ... # Use a local PLTE and global gAMA.
tRNS ... # It is transparent (maybe a logo).
IDAT ... # Note that the color type can differ
IDAT ... # from that of the other images.
IEND # End of image.
MEND # End of MNG datastream.
18.7. Example 7: A movie with sprites
Here is another movie, illustrating the use of Delta-PNG datastreams
as sprites:
\212 M N G \r \n ^z \n # MNG signature.
MHDR 512 512 30 0 0 0 1007 # Start of MNG datastream.
FRAM 2 "frame 1" 0 2 0 0 0 3 # First frame
# sets interframe_delay=3 ticks.
DEFI 1 # Define image 1 (abstract, LOCA 0 0).
IHDR 512 512 ... # It is a full-display PNG image.
etc # Chunks according to PNG spec.
IEND # SHOW 1 is implied by DEFI 1.
DEFI 2 0 1 300 200 # Define image 2, concrete.
IHDR 32 32 ... # It is a small PNG.
gAMA 50000
IDAT ...
IEND
FRAM 0 "frame 2" # Start new frame.
# New location for image 1 is still 0,0.
SHOW 1 # Display image 1 from previous frame.
MOVE 2 2 1 10 5 # New (delta) location for image 2.
SHOW 2 # Retrieve image 2 from previous frame,
CLON 2 3 0 1 0 # make a full clone of it as image 3.
0 400 500 # Location for image 3.
DHDR 3 1 7 0 0 0 0 # Modify image 3 (no change to pixels).
tRNS ... # Make it semitransparent.
IEND # SHOW 3 is implied by CLON visibility.
FRAM 0 "frame 3" # Next frame (repeat this FRAM-SHOW 1 3
# sequence with different locations to
# move the images around).
# New location for image 1 is still 0,0.
MOVE 2 2 1 10 5 # New (delta) location for image 2.
MOVE 3 3 1 5 -2 # New location for image 3.
SHOW 1 3 # Show images 1 through 3.
FRAM 0 "frame 4" # Another frame.
etc.
FRAM 0 "frame 99"
etc. # More frames.
MEND # End of MNG datastream.
18.8. Example 8: A movie with an animated sprite
This movie illustrates the use of several abstract images with
Show_mode=6 to describe an animated sprite, and the PAST chunk to turn
it around. The sprite runs back and forth across the background ten
times. The FRAM clipping boundaries restrict the screen updates to the
small region that changes, with a little "wiggle room" to make sure the
disturbed part of the background gets updated.
\212 M N G \r \n ^z \n # MNG signature.
MHDR 512 512 30 0 0 0 975 # Start of MNG datastream.
FRAM 2 "frame 1" 0 2 0 0 0 3 # First frame.
DEFI 1 IHDR 512 512 ... # Background PNG image.
etc ... IEND # Chunks according to PNG spec.
DEFI 10 1 0 x0 y0 # Static part of sprite.
IHDR 64 64 ... IDAT ... IEND
DEFI 11 1 0 x0 y1 # View 1 of animated part.
IHDR 64 32 ... IDAT ... IEND # (y1=y0+64)
DEFI 12 1 0 x0 y1 # View 2 of animated part.
IHDR 64 32 ... IDAT ... IEND
DEFI 13 1 0 x0 y1 # View 3 of animated part.
IHDR 64 32 ... IDAT ... IEND
FRAM 0 0 0 0 2 0 0 x0-dx x0+64+dx y0-dy y1+32+dy
LOOP 0 0 10
LOOP 1 0 150
FRAM 0 "left-to-right" 0 0 2 0 1 dx dx dy dy
MOVE 10 13 1 dx dy # Move animated icon {dx, dy}.
SHOW 1 SHOW 10 # Show background and static part.
SHOW 11 13 6 # Select the next view of the
ENDL 1 # animated part and show it.
FRAM SHOW 1
PAST 10 0 0 0 10 1 4 0 0 0 0 0 64 64
PAST 11 0 0 0 11 1 4 0 0 0 0 0 64 32
PAST 12 0 0 0 12 1 4 0 0 0 0 0 64 32
PAST 13 0 0 0 13 1 4 0 0 0 0 0 64 32
LOOP 1 0 150
FRAM 0 "right-to-left" 0 0 2 0 1 -dx -dx -dy -dy
MOVE 10 13 1 -dx -dy # Move animated icon {-dx, -dy}.
SHOW 1 SHOW 10 # Show background and static part.
SHOW 11 13 6 # Select the next view of the
ENDL 1 # animated part and show it.
ENDL 0 FRAM
MEND
18.9. Example 9: "Fading in" a transparent image
The opaque parts of this image will "fade in" gradually. This
example also illustrates the use of the
PPLT
and
fPRI
chunks.
\212 M N G \r \n ^z \n # MNG signature.
MHDR 64 64 30 0 0 0 1007 # Width, height, ticklength, ....
BACK 52800 52800 52800 # "Browser gray" default background.
FRAM 3 0 2 0 0 0 3 # Set interframe_delay=3 ticks. Use
# framing mode 3 so background gets restored.
DEFI 1 1 1 # Invisible and "concrete".
IHDR ... # PNG header.
PLTE ...
tRNS 0 # Entries are zero for the transparent (0)
# color and 255 for the nontransparent ones.
IDAT ...
IEND
fPRI 0 0 # Give the fade-in sequence a low priority.
CLON 1 2 # Make a working concrete copy of the image
# that will be modified during the low-priority
# part of the datastream. It is a full clone.
DHDR 2 1 7 # No change to pixel data.
tRNS 0 0 0 0 0 0 ... # Make all pixels fully transparent.
IEND
SHOW 2 2 3 # Make it visible but do not show it now.
LOOP 0 0 15
DHDR 2 1 7 # A Delta-PNG.
# Delta-type 7 means no change to pixels.
PPLT 1 10 3 16 16 16 16 ... # Increment all alphas except
IEND # for entry 0 by 16.
SHOW 2
ENDL 0 # Nontransparent pixel alpha=15, 31, ... 240.
DISC 2 # Discard the working copy.
fPRI 0 255 # Give the final frame the highest value
FRAM 0 0 1 0 0 0 60 # Hold the last frame for at least
# 60 ticks (2 sec). Applications might show it longer.
SHOW 1 # This copy still has alpha=255 for the
# opaque pixels and alpha=0 for the others.
MEND # End of MNG.
18.10. Example 10: Storing three-dimensional images
In this example, we store a series of twenty-four 150 x 150 x 150
blocks of eight-bit voxels. Each block is stored as a composite frame with
the first image being a PNG whose pixels represent the top layer of
voxels, which is followed by 149 Delta-PNG images representing the rest
of the layers of voxels. Only one image is defined, through which the
parent image is passed along from PNG to Delta-PNG to Delta-PNG. This
example also illustrates the use of unregistered ancillary chunks that
describe the x, y, and z scales and pixel calibration.
\212 M N G \r \n ^z \n # MNG signature.
MHDR 150 150 1 # Width, height, ticklength.
0 0 0 615 # Layers, frames, time, simplicity.
tEXtTitle\0Weather modeling results
tEXtComment\0The xxSC, yySC, zzSC, and ttSC chunks
in this file are written according to the Proposed
Chunk Specifications version 19970203 and Sci-Vis
Chunks Specification version 19970203 available at
ftp://swrinde.nde.swri.edu/pub/png-group/documents/
xxSC kch\0 [sig\0] kilometers\0 0\0 150
yySC kch\0 [sig\0] kilometers\0 0\0 150
zzSC kch\0 [sig\0] Height (kilometers)\0 0\0 15
ttSC kch\0 [sig\0] Time (hours)\0 0\0 24
pCAL kch\0 0 255 0 2 Degrees Celsius\0 0\0 45
DEFI 1 0 1 # All images will have image = 1
SAVE # and be visible and "concrete".
SEEK
FRAM 2 # Initial composite image.
IHDR 150 150 16 # Width, height, bit depth for top layer.
0 0 0 0 # Color, comp, filter, interlace.
IDAT ...
IEND # No DEFI chunk, so it is image 0.
DHDR 1 1 0 # Source=0, PNG, pixel addition,
150 150 0 0 # Block is entire image.
IDAT ... # IHDR is omitted; everything matches top.
IEND # IEND is also omitted.
etc. # Repeat DHDR through IEND 148 more times.
SEEK
FRAM # End of first block.
etc. # Repeat FRAM through SEEK 19 more times.
SEEK
MEND # End of MNG.
18.11. Example 11: Tiling
Here is another composite frame, illustrating the use of the LOOP
syntax to tile a large (1024 by 768) image area with a small (128 by 64)
image.
\212 M N G \r \n ^z \n # MNG signature.
MHDR 1024 768 0 # Start of MNG datastream.
98 1 0 975 # Layers, frames, time, simplicity.
FRAM 2
DEFI 1 1 0 0 -64 # Set up an offscreen "abstract" copy
IHDR 128 64 ... PLTE ... IDAT ... IEND # of the tile.
LOOP 0 0 12 # Y loop -- make 12 rows of tiles.
MOVE 1 1 1 0 64 # Move the first copy down 64 rows.
SHOW 1 # Display it.
CLON 1 2 1 # Create a partial clone of the tile.
LOOP 1 0 7 # X loop - 7 additional columns.
MOVE 2 2 1 0 128 # Move it to the right 128 columns.
SHOW 2 # Use the second copy.
ENDL 1
ENDL 0
MEND
Here is a better approach, which creates a reusable tiled image by
means of the
PAST
chunk.
\212 M N G \r \n ^z \n # MNG signature.
MHDR 1024 768 0 # Start of MNG datastream.
3 1 0 975 # Layers, frames, time, simplicity.
DEFI 1 1 # Set up an offscreen "abstract" copy
IHDR 128 64 ... PLTE ... IDAT ... IEND # of the tile.
DEFI 2 # The abstract, visible, viewable image to
BASI 1024 768 8 2 0 0 0 0 0 0 0 1 # be tiled. Initially
IEND # all pixels are zero.
PAST 2 0 0 0 # Destination and target location.
# src mod orient offset clipping
1 0 8 0 0 512 0 0 1024 0 768
# End of PAST chunk data.
MEND
18.12. Example 12: Scrolling
Here is an example of scrolling a 3000-line-high image (perhaps an
image of some text, but could be anything) through a 256-line-high
window with an alpha-blended border.
\212 M N G \r \n ^z \n # MNG signature.
MHDR 512 256 30 # Width, height, ticks per second
6513 3257 3257 975 # Layers, frames, time, simplicity.
BACK 50000 50000 50000 0 # advisory gray background
DEFI 1 1 0 0 256 # Define image 1 but do not display now.
# Initially it is offscreen, just
# below the 512 by 256 window.
IHDR 512 3000 1 0 ... # A PNG datastream containing the
PLTE ... # text (or whatever) to be scrolled.
IDAT ...
IEND
DEFI 2
IHDR 512 256 8 6 ... # A PNG datastream containing some kind
PLTE ... # of alpha-blended border that is
tRNS ... # transparent in the center.
IDAT ...
IEND
LOOP 0 0 3256
MOVE 1 1 1 0 -1 # Jack image 1 up one scanline, 3256 times.
# It ends up just above the 512 by 256 window.
# The border does not move.
FRAM 1 0 2 0 0 0 0 # Interframe_delay = 0 ticks.
# We use Framing_mode=1 to avoid unnecessary
# screen clearing between frames.
SHOW 1 # Show first image and continue without delay.
FRAM 1 0 2 0 0 0 1 # Interframe_delay = 1 tick.
SHOW 2 # Composite second image over first, wait 1 tick.
ENDL 0
MEND
Alternatively, we can declare the scrolling object to be the
background and use framing-mode 3:
(Same as above down to the LOOP chunk.)
BACK 50000 50000 50000 2 1 # Advisory gray background.
# Mandatory image background.
FRAM 3 0 2 0 0 0 1 # Interframe_delay = 1 tick.
LOOP 0 0 3256
MOVE 1 1 1 0 -1 # Jack background up one scanline, 3256 times.
SHOW 2 # Composite the second image over it, wait 1 tick.
ENDL 0
MEND
18.13. Example 13: Cycling animations
This demonstrates the use of the
SHOW
chunk with
show_mode=6
to create animations that cycle through
a series of ten objects.
This will cycle through the ten objects in the forward direction,
100 times, unless terminated sooner by the user or the decoder.
\212 M N G \r \n ^z \n # MNG signature.
MHDR 400 88 30 # Width, height, ticks per second
11 1001 1001 583 # Layers, frames, time, simplicity.
DEFI 1 ...
etc. # Define 10 objects.
DEFI 10 ...
LOOP 0 100 6 # 100 iterations, user-discretion, cacheable
SHOW 1 10 6
ENDL 0
MEND
This will cycle through the ten objects, back and forth,
50 times, unless terminated sooner by the user or the decoder.
\212 M N G \r \n ^z \n # MNG signature.
MHDR 400 88 6 # Width, height, ticks per second
11 901 901 583 # Layers, frames, time, simplicity.
DEFI 1 ...
etc. # Define 10 objects.
DEFI 10 ...
CLON 11 9 1 # Make partial clones of objects 2-9
etc. # in reverse order, as objects 11-18.
CLON 18 2 1
LOOP 0 50 6 # 50 iterations, user-discretion, cacheable
SHOW 1 18 6
ENDL 0
MEND
18.14. Example 14: Converting a GIF animation
Outline of a program to convert GIF animations to MNG format:
begin
write "MHDR" chunk
saved_images := 0; Interframe_delay := 0
First_frame := TRUE
if(loops>1) "write TERM 3 0 0 loops" chunk
write "BACK" chunk
for subimage in gif89a file do
if(interframe_delay != gif_duration) then
interframe_delay := gif_duration
write "FRAM 4 0 2 2 0 2 0 interframe_delay 0" chunk
First_frame := FALSE
else if(First_frame == TRUE)then
write "FRAM 4" chunk
First_frame := FALSE
else
write "FRAM" chunk
endif
if(X_loc == 0 AND Y_loc == 0) then
write "DEFI saved_images 1 1" chunk
else
write "DEFI saved_images 1 1 X_loc Y_loc" chunk
write "
write "SHOW 0 saved_images" chunk
if (gif_disposal_method == 0
OR gif_disposal_method == 2) then
/* (undefined or restore background) */
write "DISC" chunk
saved_images := 0
else if (gif_disposal_method == 1) then
/* (keep) */
saved_images := saved_images + 1
else if (gif_disposal_method == 3) then
/* (restore previous) */
write "DISC saved_images" chunk
endif
endfor
write "FRAM" and "MEND" chunks
end
Where "
frame converted to PNG format.
Caution: if you write such a program, you might have to pay royalties
in order to convey it to anyone else.
18.15. Example 15: Converting a simple GIF animation
Outline of a program to convert simple GIF animations that do not
use the "restore-to-previous" disposal method to "simple" MNG
(or "MNG-LC") format:
begin
write "MHDR" chunk
Interframe_delay := 0; Previous_mode := 1
Framing_mode := 1
if(loops>1) "write TERM 3 0 0 loops"
write "mandatory BACK" chunk
for subimage in gif89a file do
if(interframe_delay != gif_duration) then
interframe_delay := gif_duration
write "FRAM 0 0 2 2 0 2 0 interframe_delay 0"
endif
if(X_loc != 0 OR Y_loc != 0) then
write "DEFI 0 0 0 X_loc Y_loc" chunk
endif
write "
if (gif_disposal_method < 1) then
/* (none or keep) */
Framing_mode := 1
else if (gif_disposal_method == 2) then
/* (restore background) */
write "FRAM 4 0 1 0 1 0 0 L R T B"
Previous_mode := 4; Framing_mode := 1
else if (gif_disposal_method == 3) then
/* (restore previous) */
error ("can't do gif_disposal method = previous.")
endif
if(Framing_mode != Previous_mode) then
write "FRAM Framing_mode" chunk
Previous_mode := Framing_mode
endif
end
write "MEND" chunk
end
Where "
frame that has been converted to PNG format.
Caution: if you write such a program, you might have to pay royalties
in order to convey it to anyone else.
18.16. Example 16: Counting layers and frames
This demonstrates the determination of the layer count and frame count
that should be written in the
MHDR
chunk.
For framing_modes 1 and 2, the
FRAM
chunks themselves
do not generate layers. For framing_modes 3 and 4, they do
generate layers ("B" for background), and also generate frames if
there is no embedded image with which to combine the background layer.
Note that every framing_mode creates a "B" layer at the beginning.
Given the following chunk stream:
MHDR sRGB Fn F I I I F F I I I F F I I I MEND
in which
Fn represents a FRAM chunk with framing_mode n
F represents an empty FRAM chunk;
I represents an embedded image
This table shows the layer count and frame count for each of
the four possible values of framing-mode:
Framing Layer count Frame count
mode
1 B,I,I,I, I,I,I, I,I,I = 10 BI,I,I, I,I,I, I,I,I = 9
2 B,I,I,I, I,I,I, I,I,I = 10 BIII,III,III = 3
3 3*(B, B,I, B,I, B,I) = 21 3*(B,BI,BI,BI) = 12
4 3*(B,B,I,I,I) = 15 B,BIII,B,BIII,B,BIII = 6
18.17. Example 17: Storing an icon library
Here is an example of storing a library of icons in a
MNG-LC datastream.
All of the icons use the same palette, transparency, and colorspace,
so these are put in global chunks at the beginning. Empty
PLTE
chunks in the embedded images are used to import the global palette and
transparency data.
MHDR 96 96 1 6 5 5 459 # Profile 459 is MNG-LC
sRGB 2 # Global sRGB
PLTE ... # Global PLTE
tRNS 0 # Global tRNS
eXPI 0 "thumbnail"
IHDR 32 32 ... PLTE IDAT ... IEND
eXPI 0 "left arrow"
IHDR 96 96 ... PLTE IDAT ... IEND
eXPI 0 "right arrow"
IHDR 96 96 ... PLTE IDAT ... IEND
eXPI 0 "up arrow"
IHDR 96 96 ... PLTE IDAT ... IEND
eXPI 0 "down arrow"
IHDR 96 96 ... PLTE IDAT ... IEND
MEND
This is similar, but it uses Delta PNG datastreams to create modified
versions by replacing the palette. This can be more storage-efficient,
but requires a full MNG decoder because of the presence of Delta PNG
datastreams.
MHDR 96 96 1 6 5 5 1007 # Profile 1007 is MNG without JNG
sRGB 2 # Global sRGB
PLTE ... # Global PLTE
tRNS 0 # Global tRNS
eXPI 0 "thumbnail"
IHDR 32 32 ... PLTE IDAT ... IEND
SEEK "left arrows"
DEFI 1
IHDR 96 96 ... PLTE IDAT ... IEND
eXPI 1 "red left arrow"
DHDR 1 1 7 PPLT ... IEND # Change some palette entries.
eXPI 1 "blue left arrow"
SEEK "right arrows"
IHDR 96 96 ... PLTE IDAT ... IEND
eXPI 1 "red right arrow"
DHDR 1 1 7 PPLT ... IEND
eXPI 1 "blue right arrow"
MEND
18.18. Example 18: MAGN methods
This demonstrates the methods used in the
MAGN
chunk.
Original 3x2 object or embedded image:
1 9 1
9 17 9
Magnification method 1, XM = 5, YM = 3.
Replicates each pixel 4 additional times in
the X direction and 2 additional times in the
Y direction; new size is 15x6:
1 1 1 1 1 9 9 9 9 9 1 1 1 1 1
1 1 1 1 1 9 9 9 9 9 1 1 1 1 1
1 1 1 1 1 9 9 9 9 9 1 1 1 1 1
9 9 9 9 9 17 17 17 17 17 9 9 9 9 9
9 9 9 9 9 17 17 17 17 17 9 9 9 9 9
9 9 9 9 9 17 17 17 17 17 9 9 9 9 9
Magnification method 2, XM = 8, YM = 4.
Fills the X intervals with 7 new pixels
and the Y interval with 3 new pixels
and interpolates to get pixel values;
new size is 17x5:
1 2 3 4 5 6 7 8 9 8 7 6 5 4 3 2 1
3 4 5 6 7 8 9 10 11 10 9 8 7 6 5 4 3
5 6 7 8 9 10 11 12 13 12 11 10 9 8 7 6 5
7 8 9 10 11 12 13 14 15 14 13 12 11 10 9 8 7
9 10 11 12 13 14 15 16 17 16 15 14 13 12 11 10 9
Magnification method 3, XM = 8, YM = 4
Fills the X intervals with 7 new pixels
and the Y interval with 3 new pixels
replicating the closest pixel to get
pixel values; new size is 17x5:
1 1 1 1 1 9 9 9 9 9 9 9 9 1 1 1 1
1 1 1 1 1 9 9 9 9 9 9 9 9 1 1 1 1
1 1 1 1 1 9 9 9 9 9 9 9 9 1 1 1 1
9 9 9 9 9 17 17 17 17 17 17 17 17 9 9 9 9
9 9 9 9 9 17 17 17 17 17 17 17 17 9 9 9 9
18.19. Example 19: MAGN chunks and ROI
This example demonstrates the use of MNG to display a region of interest
(ROI) at a higher quality than the rest of the frame, and the
MAGN
chunk to convey a highly-compressed but very lossy image, a drop shadow,
and a diagonal gradient background.
MHDR 600 600 0 5 1 0 83
# Gradient background
MAGN 00 00 2 599
sRGB 1
IHDR IDAT IEND
# Drop shadow
DEFI 0 0 0 52 52
BASI 512 512 1 4 0 0 0 51 51 51 153 1
IEND # Grey-Alpha object, 46 bytes
# Main image, with most of the region of interest
# replaced with a solid rectangle, and reduced to
# 128x128 dimensions, low quality JPEG compression.
DEFI 0 0 0 36 36
MAGN 00 00 2 04 04 06 05 06 05
JHDR 128 128 10 8 8 0 0 0 0 0
JDAT
IEND
# Region of interest, full scale, cropped to
# dimensions 200x313 at location 192,200,
# high quality JPEG compression.
MAGN # Turn off magnification of all subsequent object 0
DEFI 0 0 0 228 236
JHDR 200 312 10 8 8 0 0 0 0 0
JDAT
IEND
MEND
For this image, the resulting 600x600 frame occupies about 2.6
times the file size when written as a simple JNG and about 26 times the
file size when written as a simple PNG. The particular image used in
this example was the 512x512 color Lena from
19. Credits
Editor
Glenn Randers-Pehrson,
randeg @ alum.rpi.edu
Contributors
Contributors' names are presented in alphabetical order:
Mark Adler
madler @ alumni.caltech.edu
Matthias Benkmann,
mbenkmann @ gmx.de
Thomas Boutell
boutell @ boutell.com
John Bowler,
jbowler @ acm.org
Christian Brunschen
christian @ brunschen.com
Glen Chapman,
glenc @ clark.net
Adam M. Costello
amc @ cs.berkeley.edu
Lee Daniel Crocker
lee @ piclab.com
Peter da Silva,
peter @ starbase.neosoft.com
Andreas Dilger
adilger @ turbolinux.com
Oliver Fromme
oliver @ fromme.com
Jean-loup Gailly,
jloup @ gzip.org
Chris Herborth
chrish @ pobox.com
Alex Jakulin,
jakulin @ acm.org
Gerard Juyn,
gjuyn @ xs4all.nl
Neal Kettler
neal @ westwood.com
Tom Lane,
tgl @ sss.pgh.pa.us
Alexander Lehmann,
lehmann @ usa.net
Chris Lilley
chris @ w3.org
Dave Martindale,
davem @ cs.ubc.ca
Carl Morris,
msftrncs @ htcnet.com
Owen Mortensen,
ojm @ acm.org
Josh M. Osborne,
stripes @ va.pubnix.com
Keith S. Pickens,
ksp @ rice.edu
Glenn Randers-Pehrson,
randeg @ alum.rpi.edu
Nancy M. Randers-Pehrson,
randeg @ alum.rpi.edu
Greg Roelofs
newt @ pobox.com
Willem van Schaik
willem @ schaik.com
Guy Schalnat,
gschal @ infinet.com
Paul Schmidt,
pschmidt @ photodex.com
Smarry Smarasderagd,
smar @ reptiles.org
Alaric B. Snell,
alaric @ alaric-snell.com
Thomas R. Tanner,
ttehtann @ argonet.co.uk
Cosmin Truta
cosmin @ cs.toronto.edu
Guido Vollbeding,
guivol @ esc.de
Tim Wegner,
twegner @ phoenix.net
Trademarks
GIF is a service mark of CompuServe Incorporated.
PostScript is a trademark of Adobe Systems.
X Window System is a trademark of the Massachusetts Institute of
Technology.
Document source
This document was built from the file
mng-master-20010209
on 09 February 2001.
Copyright Notice
Copyright © 1998-2001, by Glenn Randers-Pehrson
This specification is being provided by the copyright holder
under the following license. By obtaining, using and/or copying this
specification, you agree that you have read, understood, and will comply
with the following terms and conditions:
Permission to use, copy, and distribute this specification for any
purpose and without fee or royalty is hereby granted, provided that the
full text of this
NOTICE
appears on
ALL
copies
of the specification or portions thereof, including modifications, that
you make.
THIS SPECIFICATION IS PROVIDED "AS IS," AND
COPYRIGHT HOLDER
MAKES NO REPRESENTATIONS OR WARRANTIES, EXPRESS OR IMPLIED. BY WAY OF
EXAMPLE, BUT NOT LIMITATION, COPYRIGHT HOLDERS MAKE NO REPRESENTATIONS
OR WARRANTIES OF MERCHANTABILITY OR FITNESS FOR ANY PARTICULAR PURPOSE
OR THAT THE USE OF THE SPECIFICATION WILL NOT INFRINGE ANY THIRD PARTY
PATENTS, COPYRIGHTS, TRADEMARKS OR OTHER RIGHTS. COPYRIGHT HOLDER WILL
BEAR NO LIABILITY FOR ANY USE OF THIS SPECIFICATION.
The name and trademarks of copyright holder may
NOT
be
used in advertising or publicity pertaining to the specification
without specific, written prior permission. Title to copyright in this
specification and any associated documentation will at all times remain
with copyright holder.
End of MNG Specification.